feat: describe extension mechanism based on DIDs

[awoie]

This PR describes the extension point for ecosystem-specific issuer key resolution based on DIDs.

Preview here https://drafts.oauth.net/oauth-sd-jwt-vc/awoie/add-extension/draft-ietf-oauth-sd-jwt-vc.html

[awoie]

Supersedes #294

[peacekeeper]

I don't really understand the weird politics and business interests behind trying SO hard again and again to remove an OPTIONAL feature. Why is this so important to some? What kind of interests lead editors to try to sneak in changes, to avoid discussion, and to ignore consensus? What does this change accomplish?

The fact is that a lot of people have already implemented this and have expressed support for DIDs in this specification (e.g. see #250).

I don't see the point of delegating this to "separate specifications or ecosystem regulations", this will only overcomplicate things and reduce interoperability.

I know that the "hopes and dreams of the DID community" are "tiresome" to some people. So why not just leave things as they are now and move on to something else.