Unclear where the IANA registered value is used for Proposals, Wire Formats, Credentials

[rohanmahy]

It is unclear in Capabilities in LeafNodes and in the GroupContext where non-GC extension extension IDs are advertised.

[rohanmahy]

The second paragraph of Section 2 of mls-extensions ended: "so that the only value an extension developer must add to the IANA registry themselves is a unique ExtensionType."

Authors, is this unique ExtensionType supposed to:

• be a new safe extensions registry (in which case it needs to be added to the IANA considerations),
• be registered in a single existing registry regardless of the type of the extension (if so which one?), OR
• be registered in the proposals registry if it is a proposal, in the wire_formats registry if it is a wire_format, etc.?

I think we probably want the first of these, but could be convinced otherwise.