Update /credentials/{{id}} endpoint to return {verifiableCredential} to be consistent with other APIs

[dlongley]

Today the VC is directly returned -- instead it should be returned under the verifiableCredential property to be consistent with other APIs and to allow for additional meta data to be returned, etc. Similarly, we should probably make {{id}} say {{credentialId}} to match our use of that name elsewhere.

[msporny]

The group talked about this on the 2024-07-23 telecon:

@dlongley noted that all other API endpoints return verifiableCredential in the payload, except for this endpoint. We should make this endpoint be consistent with the other endpoints.

A PR should be raised to make the /credentials/{{id}} endpoint to return {verifiableCredential} to be consistent with other APIs.