Add support to express the issuer identifier as a DID

[Kristina Yasuda]

Currently the spec limits authorization server's issuer identifier to be https scheme.

There are enough implementations of OpenID4VCI that are interested in using DIDs as an issuer identifier. This would also impact discovery of an issuer metadata - from a service endpoint in a DID Document as opposed to appending .well-known to an https URL.

[Michael Jones]

This issue was mentioned during the 3-Nov-22 SIOP special topic call during discussion of https://bitbucket.org/openid/connect/issues/1632 .

[Kristina Yasuda]

I am ok to close this based on the PR discussion. - I think for now https issuer identifier is sufficient. but i reserve the right to reopen it ;)