[FEATURE] Alter refund schema to support VSAAS cases

### Feature Description

Make changes to support the integration of the (VSAAS) into the refund flow. The new columns, `processor_merchant_id` and `created_by`, are essential for storing specific information related to VSAAS transactions within the refund record.
This will allow for better tracking and management of refunds, which is a key requirement for Vertical SaaS implementations.

### Possible Implementation

Add new columns to refunds schema

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

- [x] I checked and didn't find a 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?

Yes, I am willing to submit a PR!

The issue names no files, migrations, or tests. Start by locating the refunds schema and its migration history, then inspect existing refund fields and related tests. Done means the schema stores processor_merchant_id and created_by for refund records and the relevant checks pass.

Written by the indexing model from the issue text.