[Feature]: As a korifi user, I want to be able to retrieve credentials from a user-provided service

[klapkov]

Background

The credentials endpoint for service instances is only needed for user-provided service instances. We keep these credentials in a kubernetes secret. API doc: https://v3-apidocs.cloudfoundry.org/version/3.180.0/index.html#get-credentials-for-a-user-provided-service-instance

Acceptance Criteria

• Implement GET /v3/service_instances/:guid/credentials