Add support for Client OAuth ID Metadata Document

The specification seems to still be a draft, but it's an interesting new way to register a client dynamically during the authorization phase (which, among other things, allow asking the user if it's OK to register this client).

The specification (does not seem to be final yet) seems to be https://datatracker.ietf.org/doc/draft-ietf-oauth-client-id-metadata-document/.

Note that it does not seem to be supported by oauth2-oidc-sdk yet.