Add tags list to /api/v1/cred endpoint

[ktims]

Currently the list of tags attached to a credential is not included in the JSON from the cred endpoint. As far as I can tell there's no simple way to enumerate the tags attached to a credential using the API; only a list of credentials with a tag can be obtained, not a list of tags attached to a credential.

I think in this context it makes sense to expand the list and provide the name, id and resource_uri of each tag rather than requiring a followup query for each tag.