RFC 0453 - missing credentials attachment registry for non-hlindy-zkp-v1.0 (specifically W3C JWT/LD-Proofs)

RFC [0511](https://github.com/hyperledger/aries-rfcs/issues/538) notes:
> The Issuer SHOULD set the format to the value that corresponds to the format the credentials are issued in.

Currently, [0453](https://github.com/hyperledger/aries-rfcs/tree/master/features/0453-issue-credential-v2) does not support other credential formats than `hlindy-zkp-v1.0`.

@llorllale I guess we should add support for a W3C VC format to the registry. We could also add more granular formats that distinguishes between W3C VC JWT and W3C VC LD-Proofs. Not sure if a further distinction between VCs that contain DIDs in the `@id` property of the credentialSubject is required. Ideally, there is an existing registry that we can link such as [DIF Credential Format Designations](https://identity.foundation/presentation-exchange/#credential-format-designations).