Define a capabilities mechanism

[pthatcherg]

We need a way to know which parts of the protocol an endpoint supports: does it allow Presentations? Remote Playback? Streaming? And if we make a new version of a protocol, does it support that? We need a mechanism for this.

[rektide]

I don't know if this capability will quite fit in, but I'd be interested in devices being able to indicate they are audio or video only.

[mfoltzgoogle]

Should ensure that any capabilities disclosed before authentication are discussed in the Security & Privacy section, and the possible implications thereto.

[rektide]

My ask for audio/video capabilities appears to be the primary thing implemented in the capabilities PR, #140 . 👍

[mfoltzgoogle]

Closing this as we've landed spec changes:

• For agents to exchange capabilities
• A list of capabilities defined by the spec
• A way for agents to advertise extended (custom) capabilities
• A registry of extended capabilities
• Noting that capabilities are advertised to any network client and are public information.

The remaining open issue around capabilities is #200, but that is specific to receives-audio and receives-video. Closing this issue for now based on the current state of the spec.