OpenID Connect Dynamic Client Registration

[azmeuk]

This implements OpenID Connect Dynamic Client Registration and fixes #705.

Basically, a new claims class is created at authlib/oidc/registration/claims.py with all the missing claims.
On the same time I have tuned RegistrationEndpoint so it can handle several claims at the same time (this is also in prevision of #500 and the additional post_logout_redirect_uris registration claim).

Here is how one can use OIDC Dynamic Client Registration:

    from authlib.oauth2.rfc7591 import ClientMetadataClaims as OAuth2ClientMetadataClaims
    from authlib.oauth2.rfc7591 import ClientRegistrationEndpoint
    from authlib.oidc.registration import ClientMetadataClaims as OIDCClientMetadataClaims

    class MyClientRegistrationEndpoint(ClientRegistrationEndpoint):
        ...

        def get_server_metadata(self):
            ...

    authorization_server.register_endpoint(
        MyClientRegistrationEndpoint(
            claims_classes=[OAuth2ClientMetadataClaims, OIDCClientMetadataClaims]
        )
    )

[lepture]

LGTM