Issue:
Feat: Ability to accept 3 values in ConnectorAuthType

Currently we support max 2 values from keys.conf / db
```rust
pub enum ConnectorAuthType {
    HeaderKey { api_key: String },
    BodyKey { api_key: String, key1: String },
}
```

I'm trying to integrate Cybersource, it requires [`http signatures`](https://developer.cybersource.com/docs/cybs/en-us/payments/developer/all/rest/payments/GenerateHeader/httpSignatureAuthentication.html#id193AL0O0BY4_id199IL0O0AY4). 

I will need 3 values
 * Merchant Id
 * Api Key
 * Api Secret
 
 With these I'm able to generate the http signature headers, like
 ```
 digest: "SHA-256=cwjLNSMNo0IFp7hbUtTNu+7KxaF9O67ydqKWMnQ7J5g="
 signature: 'keyid="5476633e-eff2-4e65-9834-58081207dd61", algorithm="HmacSHA256", headers="host (request-target) digest v-c-merchant-id", signature="djnWLdaLRh8xtWLCXxGIlavyRG4jBvB7gIzUWTKzPoQ="'
```

So I'm proposing 

```rust
pub enum ConnectorAuthType {
    HeaderKey { api_key: String },
    BodyKey { api_key: String, key1: String },
    SignatureKey { api_key: String, key1: String, api_secret: String },
}
```

@SanchithHegde @Narayanbhat166 @jarnura 

I can add the PR for it.

referring #58 

Resolution:
<think>
PR Context:
- Title: feat(router): add support for Signature Key ConnectorAuthType
- Merged: 2022-12-08T09:11:28Z
- Files changed: 1 files
- Commits: 2 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates

## Description
This will allow us to accept 3 keys for a connector.
Generally Public key, Private key and the merchant id.

<!-- Describe your changes in detail -->


### Additional Changes

- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!-- 
Provide li
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(router): add support for Signature Key ConnectorAuthType (#85)
Co-authored-by: Arun Raj M <jarn
Files to modify:


**crates/router/src/types.rs**
  Modify existing code