[ed] move parts that are relevant to both specifications to OpenID4VP from SIOPv2

[Kristina Yasuda]

Taking into account how the specifications have evolved, discussions with the editors, and feedback from the implementers, it seems that features common to both specifications (mainly non-pre-registered client metadata resolution and cross-device flow description) should sit in OpenID4VP specification - in many use-cases OpenID4VP is used on its own to transport verifiable credentials, with SIOPv2 added is user authentication is required.

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[Kristina Yasuda]

PR #354 merged. the process of moving will continue as part of the prep for ID2