feat(mgmt): add credit purchase endpoints

[jvallesm]

Because

• Instill Cloud must allow for credit purchase & purchase history.

This commit

• Exposes the data required in the purchase page via 2 endpoints:
  • Get remaining credit returns detailed information about remaining credit.
  • List purchases returns a paginated list of credit purchases.

[linear]

INS-4210 Purchase credit

[jvallesm]

Removed credit purchase list from the PR as it will be handled by the frontend in the beginning. I'll add a separate PR with the endpoint that can be opened when we tackle the switch from backend to frontend.