Issue:
refactor(authentication): authenticate merchant by API keys from API keys table

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates

## Description
<!-- Describe your changes in detail -->
This PR updates the API key authentication code to use the API keys from the API keys table instead of the merchant account table. I'll be taking up the deprecation of the `api_key` field from the merchant account table in a separate PR.

## Motivation and Context
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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 its an obvious bug or documentation fix
that will have little conversation).
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This is an extension of PRs #511, #639 and #705, thus completely addressing #501.

## 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)?
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Manually. After I made the change, I created a new merchant account and API key, and tested out creating customers and payments. Things worked as expected.

## 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 submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable

---

~Marking as this as draft as this is blocked by #639 and #705 being merged to `main` branch.~

Resolution:
<think>
PR Context:
- Title: chore(merchant_account): remove `api_key` field
- Merged: 2023-03-13T09:20:25Z
- Files changed: 14 files
- Commits: 6 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates

## Description
<!-- Describe your changes in detail -->
This PR removes the unused `api_key` field from the merchant account table and request/response schemas.

### Additional Changes

- [x] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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Provide links to t
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** chore(merchant_account): remove `api_key` field (#713)
Files to modify:


**crates/api_models/src/admin.rs**
  Modify existing code

**crates/router/src/core/admin.rs**
  Modify existing code

**crates/router/src/db/merchant_account.rs**
  Remove:
    - function: private::find_merchant_account_by_api_key
    - function: private::find_merchant_account_by_api_key

**crates/router/src/services/api.rs**
  Remove:
    - function: pub::authenticate_by_api_key

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

**crates/router/tests/connectors/globalpay.rs**
  Modify existing code

**crates/router/tests/payments.rs**
  Modify existing code

**crates/router/tests/payments2.rs**
  Modify existing code

**crates/storage_models/src/merchant_account.rs**
  Modify existing code

**crates/storage_models/src/query/merchant_account.rs**
  Remove:
    - function: pub::find_by_api_key

**crates/storage_models/src/schema.rs**
  Modify existing code