Return also redeem endpoint as client registration response

[federicaagostini]

As per RFC 7591, the client registration metadata may contain additional fields:

   Extensions and profiles of this specification can expand this list
   with metadata names and descriptions registered in accordance with
   the IANA Considerations in Section 4 of this document.

Thus, we can add a redeem URI key (e.g. reedem_client_uri) with value https://<iam-host>/iam/api/client-registration/<client-uuid>/redeem to indicate the endpoint used to link a client to an account. (Rephrased from oidc-agent devs:) This would also allow oidc-agent to link the client to the user who authenticates.