Issue:
 Implement `EphemeralKeyInterface` for `MockDb`

Spin out from https://github.com/juspay/hyperswitch/issues/172. Please refer to that issue for more information.

Resolution:
<think>
PR Context:
- Title: feat(db): implement `EphemeralKeyInterface` for `MockDb`
- Merged: 2023-06-25T05:52:25Z
- Files changed: 3 files
- Commits: 6 commits

Implementation approach:
## Type of Change


- [ ] Bugfix
- [X] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
Implemented Mock db for ephemeral key


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables



## Motivation and Context

Fixes #1191 

## How did you test it?


## Checklist

- [ ] 
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(db): implement `EphemeralKeyInterface` for `MockDb` (#1285)
Files to modify:


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

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

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