Issue:
Implement `MandateInterface` 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 `MandateInterface` for `MockDb`
- Merged: 2023-06-17T08:25:58Z
- Files changed: 2 files
- Commits: 25 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [x] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
- fixes #1256 

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with co
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(db): implement `MandateInterface` for `MockDb` (#1387)
Files to modify:


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

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