Make OpenID4VP over the Digital Credentials API more generic

[fkj]

The OpenID4VP over the Digital Credentials API section currently has some wording that makes assumptions about how native App Platform DC API equivalents in operating systems are implemented.
Since there are very few of these currently in existence and it is not clear that they will all work the same way, we should try to make the text more generic.

In particular:

• There is some text talking about how the origin of the verifier is "transported", but some platforms might just generate origins directly at each end.
• The text about use of platform-specific application origins introduced in Add security guidance for platform-specific Origins and DC API. #761 should be made more generic if possible