[FEATURE] add payment method filter in v2

### Feature Description

List all the available currencies and countries for the given connector and payment method type.

### Possible Implementation

Add the filter route to the V2 PaymentMethods implementation.

### 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!
<think>
PR Context:
- Title: feat(payment_methods): add payment-methods/filter endpoint to list countries and currencies for connector payment method in v2
- Merged: 2025-08-22T13:26:14Z
- Files changed: 2 files
- Commits: 4 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 -->
Add the payment method filter route to the V2 PaymentMethods implementation which list all available countries and currencies based on connector and payment method type.


### Additional Changes

- [X] This PR modifies the API contract
-
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** feat(payment_methods): add payment-methods/filter endpoint to list countries and currencies for conn
Files to modify:


**crates/router/src/routes/app.rs**
  Modify existing code

**crates/router/src/routes/payment_methods.rs**
  Add:
    - function: pub::list_countries_currencies_for_connector_payment_method
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