Issue:
refactor(merchant_account): add back `api_key` field for backward compatibility

## 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 adds back the `api_key` field in the `merchant_account` table temporarily. This change is a temporary one which would be rolled back once all our teams have migrated their applications/services to use the `api_keys` table and endpoints.

### Additional Changes

- [x] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
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The migrations can be found at [`up.sql`](https://github.com/juspay/hyperswitch/blob/reintroduce-merchant-account-api-key/migrations/2023-03-16-132604_merchant_account_temp_add_api_key/up.sql) and [`down.sql`](https://github.com/juspay/hyperswitch/blob/reintroduce-merchant-account-api-key/migrations/2023-03-16-132604_merchant_account_temp_add_api_key/down.sql).

## Motivation and Context
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Backward compatibility.

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
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Verified that the same API keys are being stored in both the `merchant_account` and `api_keys` tables. 

## 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

---

_P.S.: I'd prefer not having to make this change in the first place._

Resolution:
<think>
PR Context:
- Title: revert: refactor(merchant_account): add back `api_key` field for backward compatibility (#761)
- Merged: 2023-05-10T15:34:13Z
- Files changed: 9 files
- Commits: 3 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR reverts #761 (commit 661dd48ae51493e404bfd50701674da6f0c7e26a). 

The `api_key` field was added for backwards compatibility and was a temporary change. Now that all our teams have migrated to use the `api_keys` table and endpoints, t
</think>

To implement this, you'll need to modify:


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