Function create_proof_jwt 

pub fn create_proof_jwt(
    signer: &dyn Signer,
    issuer_url: &str,
    c_nonce: &str,
    did: &str,
) -> Result<String>

Create a proof-of-possession JWT for an OID4VCI credential request.

The JWT proves the holder controls the private key associated with did.

Header: { "typ": "openid4vci-proof+jwt", "alg": "<alg>", "kid": "<did>#key-1" } Payload: { "iss": "<did>", "aud": "<issuer_url>", "iat": <now>, "nonce": "<c_nonce>" }