Add /protocols endpoint to specific exchanges.

[msporny]

This PR is an attempt to address issue #421 by adding the .../protocols endpoint to exchanges.

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[msporny]

This needs a better description, @dlongley. I'm at a loss as to what to write.

[dlongley]

Now that we've defined the interact URL, we should just point to that section in the spec and / or reuse language from it. The value for interact is an interaction URL: https://w3c-ccg.github.io/vc-api/#interaction-url-format

[TallTed]

Not arguing against @dlongley's suggestion, but this would be better than what's here.

Suggested change

	description: A URL that can be used during human-in-the-loop exchange flows.
	description: A URL that can be used during exchange flows with a human in the loop.

[msporny]

Maybe we need to explain how interact is supposed to work in this section?

[dlongley]

Or link to the interaction section? This section should say that the returned object lists all of the protocols supported by the exchange -- and that an implementation may implement an interaction URL handler by proxying/internally fetching this endpoint and returning its result to the caller.

We could use a mermaid style flowchart here to show an interaction URL being retrieved from a coordinator that internally looks up the associated exchange and fetches <exchangeId>/protocols and returns it to the caller. We can then explain that this allows the coordinator to delegate to an exchange service whereby the caller (client, e.g., digital wallet) can still use their trust in the coordinator origin -- as it serves (passes through) the protocols object.