Issue:
[FEATURE] Implement `MerchantKeyStoreInterface` for `MockDb`

### Feature Description

Currently the MerchantKeyStoreInterface is not implemented for MockDb. It is required for merchant account functions. And without this implementation, we cannot write tests for them.

### Possible Implementation

Checkout #172 for more details.

### Have you spent some time to check if this feature request has been raised before?

- [X] I checked and didn't find 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?

None

Resolution:
<think>
PR Context:
- Title: feat(db): implement `MerchantKeyStoreInterface` for `MockDb`
- Merged: 2023-07-25T07:52:19Z
- Files changed: 5 files
- Commits: 3 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [x] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR adds the `MockDb` implementation of `MerchantKeyStoreInterface`. In addition, this PR includes a fix for the `test_connector_label_cache` test in the `merchant_connector_account` module.

## Motivation and Context
<!--
Why is this 
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(db): implement `MerchantKeyStoreInterface` for `MockDb` (#1772)
Files to modify:


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

**crates/router/src/db/merchant_connector_account.rs**
  Modify existing code

**crates/router/src/db/merchant_key_store.rs**
  Modify existing code

**crates/router/src/types/storage.rs**
  Remove:
    - mod: pub::reverse_lookup
    - mod: pub::kv
  Add:
    - mod: pub::kv
    - mod: pub::merchant_key_store
    - mod: pub::reverse_lookup

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