**What**
- Introduce link between Region and PaymentProvider
- Introduce API endpoint `GET /store/regions/:id/payment-providers` for retrieving providers by region
- Add tests for both
~~Opening a draft PR to discuss a couple of implementation details that we should align on~~
**What**
Add workflow and API endpoint for creating payment sessions for a payment collection. Endpoint is currently `POST /store/payment-collection/:id/payment-sessions`. I suggested an alternative in a comment below.
Please note, we intentionally do not want to support creating payment sessions in bulk, as this would become a mess when having to manage multiple calls to third-party providers.
