[proposal] support for openid4vci-proof+jwt

[arnemileswinter]

i want to mention that chances are that i am confusing things here, it is also pretty late at night 😃

Is your feature request related to a problem? Please describe.
It appears impossible to verify openid4vci-proof+jwt that are presented to me during an openid4vc credential request as per https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0.html#name-proof-types

these docs mark iss and client_id as optional, while did-jwt demands them as required. simply injecting these values would break the signature though.

Describe the solution you'd like
the did is in the token header's kid attribute, not as an iss in the payload nor client_id, therefore an error is thrown (like in #273 )

Describe alternatives you've considered
do it without the library

Additional context

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.