[Openid-specs-fapi] Issue #844: Submit FAPI 2.0 MS IANA registrations (openid/fapi)

[panva]

New issue 844: Submit FAPI 2.0 MS IANA registrations
https://bitbucket.org/openid/fapi/issues/844/submit-fapi-20-ms-iana-registrations

Filip Skokan:

Filing this issue to track a TODO following the approval of FAPI 2.0 Message Signing Final Specification.

Much like #715 \(JARM Errata\) we need to submit the request for registration of the `response_modes` client metadata to the [OAuth Dynamic Client Registration Metadata](https://www.iana.org/assignments/oauth-parameters/oauth-parameters.xhtml#client-metadata) registry.