Add guidance on how to handle multiple presentation request entries

[MasterKale]

This PR adds initial text on A) how user agents MUST handle the presence of multiple presentation requests in options.digital.requests, and B) how verifiers SHOULD populate options.digital.requests to handle getting back a corresponding presentation response.

Closes #220.

The following tasks have been completed:

• Modified Web platform tests (link)

Implementation commitment:

• WebKit (link to issue)
• Chromium (link to issue)
• Gecko (link to issue)

Documentation and checks

• Affects privacy
• Affects security
• Pinged MDN
• Updated Explainer
• Updated digitalcredentials.dev

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Preview | Diff

[TallTed]

Suggested change

	[=user agents=] MUST only return one
	[=digital credential/presentation responses | presentation response=] for one
	of these requests.
	[=user agents=] MUST return a maximum of one
	[=digital credential/presentation responses | presentation response=] for each
	of these requests.

[TallTed]

This needs further refinement. I'm trying to capture the concept before getting the language just right. There will also need to be some language about what happens when a response that contains credentials A, B, and C is sent after receiving individual requests for credentials A, B, C, D and E, and then a new request for credential B is received. Does the first response cause the new request for B to be ignored? Or might a new response containing B, D, and E be sent? Or something else?

Suggested change

	Multiple credentials MAY be present in this one response depending on the capabilities
	of the corresponding [=digital credential/exchange protocol=].
	Multiple credentials MAY be present in one such response depending on the capabilities
	of the corresponding [=digital credential/exchange protocol=].
	Each credential SHOULD be present in only one such response, such that a response
	containing multiple credentials is considered to be a response to all individual
	requests for one of the credentials it contains.

[wseltzer]

Discussed 11 June minutes

[marcoscaceres]

Notes shouldn't include RFC2119 keywords (i.e., get rid of the SHOULD).

I don't think this even makes sense, to be honest. How they are presented is up to the user agent, not up to verifier.