fix: holder DID must match the signing key for vc+sd-jwt

[coplat]

Description of change

When presenting a vc+sd-jwt credential over OID4VP, the VP jwt's holder and iss had a different DID than the key that signed it, so UniCore+'s verification flow failed with the error:

"Invalid OID4VP Authorization Response: Presentation validation error: [the holder's id does not match the provided DID Document(s)]".

get_vp_token derived holder/iss from the algorithm negotiated with the verifier (EdDSA by default), but the Format::VcSdJwt arm signs with the credential's cnf key (ES256 here).

The fix derives holder and iss from cnf_kid inside the VcSdJwt arm.

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