OpenID4VP/SIOP/VCI: add custom schemes for different capabilities

[Torsten Lodderstedt]

Clients should know what capabilities to expect when invoking a wallet via a custom scheme.

VP: VP only
SIOP: SIOP only, SIOP + VP

We should also add text to describe how ecosystems can use their own custom schemes to represent the capabilities of their wallets.

[Kristina Yasuda]

I would like to suggest the following custom schemes?

1. openid://siopv2 for SIOP v2 (ID Token only)
2. openid://vp for OpenID4VP (VP Token only)
3. openid://siop-vp for ISOP v2 used with OpenID4VP (ID Token + VP Token)

though if the wallet only supports the second option, it will still open when the request for ID Token is included… which might lead to a not so good UX…

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[Kristina Yasuda]

PR merged