What protocols should a user agent implement?

[marcoscaceres]

In #401, I suggested we add the following:

    <p>
       A [=user agent=] MUST implement at least one of the [=digital credential/exchange protocol=] listed in the [=table of exchange protocols=].
       It is RECOMMENDED that user agent to implement all the [=digital credential/exchange protocols=] listed in the [=table of exchange protocols=]. 
    </p>
    <aside class="note" title="Checking which exchange protocols a user agent allows">
      <p>Developers can check which [=digital credential/exchange protocols=]
      a user agent allows by calling the `DigitalCredential.`{{DigitalCredential.userAgentAllowsProtocol()}} static method.
      See [[[#checking-if-protocol-is-allowed]]] for a usage example.
      </p>
    </aside>

As:

1. It forces user agents to support at least one - so at least developers know what they are going to get.
2. It strongly encourages implementing all, but it doesn't force it (we can't force it regardless because of 3 below or because of platform limitations).
3. It keeps existing and "legacy" user agents as compliant, specially in the case we add new exchange protocols in the future. For example, might be impossible to implement some protocol on a particular platform, but feasible to implement other protocols.

[timcappalli]

@leecam

[mohamedamir]

We should also discuss the cross-device story here.
Whether we would like to mandate supporting all protocols for cross-device flows. (which I think we should)

[MasterKale]

There's also the confusion of the user agent not allowing a protocol, communicated via DigitalCredential.userAgentAllowsProtocol(), but of course not knowing if hybrid would be successful. So what do we want to define what happens here?

1. The user agent won't allow use of other protocols from other devices, with wallets that do support the protocol, even over hybrid?
2. DigitalCredential.userAgentAllowsProtocol() becomes a signal as to whether the user will be prompted for presentation from a local wallet, otherwise DC API will jump straight to prompting for hybrid?

We're going to have a similar problem in here as with passkeys, especially for RPs that want to feature check and offer UI guiding the user to successfully respond to the DC API call:

• In Scenario 1, the RP may wish to show UI preceding the DC API call that says, "select a credential from your local wallet"
• In Scenario 2, the RP may wish instead to show UI that says, "use your mobile device to choose a credential"

Putting this out as food for thought while we consider the cross-device story.