Summary
Implement a new endpoint to return the list of billing_provider_ids for all subscriptions for a specified org_id and optional product_tag/billing_provider
 
Acceptance Criteria
Endpoint Details:
New GET endpoint at /api/swatch-contracts/v1/subscriptions/billing_provider_ids

Required Parameters:
org_id, string, can support one or multiple values

Optional Parameters:
product_tag, string
billing_provider, string

Headers:
x-rh-identity

Authorization:
Admin Users: If the user is an admin, the org_id in the header does not need to match the org_id in the query parameters.

Non-Admin Users: If the user is not an admin, the org_id in the header must match the org_id in the query parameters. If it does not, return a 403 Unauthorized response.

Response:
200 OK: Returns an array of objects with fields:

• product_tag
• billing_provider
• billing_provider_id

If no billing provider IDs match the specified filters, return an empty array.

Configuration:
Add an entry to the nginx reverse proxy configuration in the swatch-api to direct requests to /api/rhsm-subscriptions/v1/subscriptions/billing_provider_ids to /api/swatch-contracts/v1/subscriptions/billing_provider_ids