Issue:
Bug: [BUG] Fix failing postman tests for Paypal



### Bug Description

Postman tests for Paypal fail in the access token flow.

### Expected Behavior

All postman tests should pass

### Actual Behavior

The tests fail

### Steps To Reproduce

Run the paypal postman tests

### Context For The Bug

_No response_

### Environment

Local

### Have you spent some time checking if this bug has been raised before?

- [X] I checked and didn't find a similar issue

### Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

Yes, I am willing to submit a PR!

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [x] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR fixes a failing Paypal Postman test. The issue occurred because the `merchant_connector_id` was not being passed in the `routing` parameter in the request, which is not compatible with the access token flow currently. This PR adds the `merchant_connector_id` parameter to the request.

Additionally, @Narayanbhat166 will add compatibility support to the access token flow for cases where the `merchant_connector_id` is missing

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
-->


## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
Running all Paypal postman tests locally.
<img width="518" alt="image" src="https://github.com/juspay/hyperswitch/assets/47862918/48a79b42-2776-4021-86bb-c9137088ddc0">


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

Running all Paypal postman tests locally.
<img width="518" alt="image" src="https://github.com/juspay/hyperswitch/assets/47862918/48a79b42-2776-4021-86bb-c9137088ddc0">

Patch:
diff --git a/postman/collection-dir/paypal/event.prerequest.js b/postman/collection-dir/paypal/event.prerequest.js
index f4c9a764864..944aebeb6ee 100644
--- a/postman/collection-dir/paypal/event.prerequest.js
+++ b/postman/collection-dir/paypal/event.prerequest.js
@@ -3,25 +3,28 @@ const isPostRequest = pm.request.method.toString() === "POST";
 const isPaymentCreation = path.match(/\/payments$/) && isPostRequest;
 
 if (isPaymentCreation) {
-  try {
-    const request = JSON.parse(pm.request.body.toJSON().raw);
+    try {
+        const request = JSON.parse(pm.request.body.toJSON().raw);
+        const merchantConnectorId = pm.collectionVariables.get("merchant_connector_id");
 
-    // Attach routing
-    const routing = { type: "single", data: "paypal" };
-    request["routing"] = routing;
+        // Attach routing
+        const routing = {
+            type: "single", data: { connector: "paypal", merchant_connector_id: merchantConnectorId }
+        };
+        request["routing"] = routing;
 
-    let updatedRequest = {
-      mode: "raw",
-      raw: JSON.stringify(request),
-      options: {
-        raw: {
-          language: "json",
-        },
-      },
-    };
-    pm.request.body.update(updatedRequest);
-  } catch (error) {
-    console.error("Failed to inject routing in the request");
-    console.error(error);
-  }
-}
\ No newline at end of file
+        let updatedRequest = {
+            mode: "raw",
+            raw: JSON.stringify(request),
+            options: {
+                raw: {
+                    language: "json",
+                },
+            },
+        };
+        pm.request.body.update(updatedRequest);
+    } catch (error) {
+        console.error("Failed to inject routing in the request");
+        console.error(error);
+    }
+}
diff --git a/postman/collection-json/paypal.postman_collection.json b/postman/collection-json/paypal.postman_collection.json
index dfb52bb4ffc..58368b3b856 100644
--- a/postman/collection-json/paypal.postman_collection.json
+++ b/postman/collection-json/paypal.postman_collection.json
@@ -9,28 +9,32 @@
           "const isPaymentCreation = path.match(/\\/payments$/) && isPostRequest;",
           "",
           "if (isPaymentCreation) {",
-          "  try {",
-          "    const request = JSON.parse(pm.request.body.toJSON().raw);",
+          "    try {",
+          "        const request = JSON.parse(pm.request.body.toJSON().raw);",
+          "        const merchantConnectorId = pm.collectionVariables.get(\"merchant_connector_id\");",
           "",
-          "    // Attach routing",
-          "    const routing = { type: \"single\", data: \"paypal\" };",
-          "    request[\"routing\"] = routing;",
+          "        // Attach routing",
+          "        const routing = {",
+          "            type: \"single\", data: { connector: \"paypal\", merchant_connector_id: merchantConnectorId }",
+          "        };",
+          "        request[\"routing\"] = routing;",
           "",
-          "    let updatedRequest = {",
-          "      mode: \"raw\",",
-          "      raw: JSON.stringify(request),",
-          "      options: {",
-          "        raw: {",
-          "          language: \"json\",",
-          "        },",
-          "      },",
-          "    };",
-          "    pm.request.body.update(updatedRequest);",
-          "  } catch (error) {",
-          "    console.error(\"Failed to inject routing in the request\");",
-          "    console.error(error);",
-          "  }",
-          "}"
+          "        let updatedRequest = {",
+          "            mode: \"raw\",",
+          "            raw: JSON.stringify(request),",
+          "            options: {",
+          "                raw: {",
+          "                    language: \"json\",",
+          "                },",
+          "            },",
+          "        };",
+          "        pm.request.body.update(updatedRequest);",
+          "    } catch (error) {",
+          "        console.error(\"Failed to inject routing in the request\");",
+          "        console.error(error);",
+          "    }",
+          "}",
+          ""
         ],
         "type": "text/javascript"
       }