Allow changing database services' credentials via service variables

Question

Credentials are currently confusing for database services. There's no way to change or reset them like plugins used to have - I've seen some users changing the credential variables only to get confused when the old credentials are still live ([0](https://help.railway.app/questions/prisma-authentication-failed-against-d-e2b17289), [1](https://help.railway.app/questions/cannot-connect-to-postgres-db-after-chan-6c67b841))