Consider use of "DID method enum" spec

[peacekeeper]

At DIF, we have a mini-spec for a data structure that can enumerate which DID methods are supported. The idea is that in addition to just listing the method names themselves, there could be a need to express support for certain "features", e.g. which networks in did:indy are supported, which key types in did:key, etc.

See example here:
https://github.com/decentralized-identity/did-method-enum/blob/main/spec.md#combined-example

This "DID method enum" data structure could potentially be returned by the endpoint here that expresses support for DID methods:

• tswg-trust-registry-protocol/api/toip-tswg-trustregistryprotocol-v2.yaml

  Line 305 in a703e4c

  /lookup/vidmethods:

[andorsk]

I think this is a good idea and I love the fact it's borrowing from prior art. I will prepare a PR with this suggestion in mind.