Support did as trusted authorities for DCQL

[TimoGlastra]

Based on a comment I initially added to #479:

One issue i see for w3c VCs is that issuer based matching with DCQl can become quite complex, as you don't know whether to query ['issuer'] or ['issuer', 'id'].

So if you would use claim sets, you'd have to duplicate all claim sets. I know the WG decided against adding did as a trusted authority mechanism, but it would make issuer matching a lot easier IMO.

A similar limitation is there for AnonCreds credentials, as there is not really an issuer_did property to query on.

I think adding support for DIDs to trusted authorities would make querying for DID based issuers a lot easier across credential formats that support DIDs.