DID implementation missing

[ineiti]

The following parts of the DID are missing - I'm not sure if it makes sense to implement them or not:

• darc/darc.go::IdentityDID - Verify is not implemented
• darc/darc.go::Identity.Verify - so this is missing
• darc/darc.go::Identity.GetPublicKey - no DID public key
• darc/darc.go::ParseIdentity - no DID parsing