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

Spin out from #172. Please refer to that issue for more information.

Resolution:
<think>
PR Context:
- Title: feat(db): implement `RefundInterface` for `MockDb`
- Merged: 2023-06-03T13:51:23Z
- Files changed: 2 files
- Commits: 6 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [X] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->


### 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 corresponding c
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(db): implement `RefundInterface` for `MockDb` (#1277)
Co-authored-by: Sanchith Hegde <22217505+
Files to modify:


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

**crates/storage_models/src/refund.rs**
  Add:
    - function: pub::create_refund
    - impl: impl RefundUpdateInternal