Dynamic client registration response is not spec compliant

[matrixbot]

This issue was originally created by @tonkku107 at matrix-org/matrix-authentication-service#2848.

The OpenID spec requires that the response includes all the client metadata.

From OpenID Connect Dynamic Client Registration 1.0 Section 3.2:

Upon successful registration, the Client Registration Endpoint returns the newly created Client Identifier and, if applicable, a Client Secret, along with all registered Metadata about this Client, including any fields provisioned by the Authorization Server itself.

Currently Matrix Authentication Service only returns client_id and client_id_issued_at