[Discovery Feature] adding a `suported_capabilities` attribute to the metadata document

[identitymonk]

as per request from @baboulebou and @vladiber during WG meeting of 04/22, the metadata document shall have a dedicated attribute to allow the policy decision point to expose which additional capabilities it supports.

The value should be registered at IANA.

[davidjbrossard]

See also #336

[davidjbrossard]

I can see the IANA section in the spec but I don't believe this has been submitted to IANA as yet. Gerry will check and report back at the next meeting

[davidjbrossard]

Punting this to the very final stage before we move to 1.0 adoption. @ggebel will drive the process from the IANA side (see https://github.com/protocol-registries/well-known-uris?tab=readme-ov-file)

[identitymonk]

@davidjbrossard I talked to IANA at IETF 123 / Madrid cause one of my personal Draft got some IANA registry intents.

they like to be able to discuss those beforehand as it can smooth the process downstream

[identitymonk]

Note that this enhancement was part of PR 355