Add resource set registration endpoint

[njlie]

• POST /resource
• RS should identify itself with a key rather than a string or a kid
• Return instance_id that may be used to identify an RS instead of a key or by reference during token introspection.

https://datatracker.ietf.org/doc/html/draft-ietf-gnap-resource-servers#section-3.4

[wilsonianb]

Would this mean:

• the set of resources supported at the AS is dynamic
  or
• custom subsets of a known set (Open Payments) resource can be enabled?

It seems like we'd have to strip out a lot from current AS for the former. And I'm not sure if/how our custom resource access limits are registered in either case.

[njlie]

I think what I envisioned strays closer to the latter. For our purposes we really only need this endpoint to allow an RS to acquire an identifier that points to a key the AS can use to authenticate signatures on an introspection request, so we could even forgo custom resource access limit registration entirely.

[njlie]

Closing as a consequent of the reasoning in #576.

Closing for now due to AS-RS being in a trusted zone