Function prove_by_key 

fn prove_by_key(
    state: &AppState,
    token: &str,
    iss: &str,
    lxm: &str,
    now: OffsetDateTime,
) -> Result<Proof, ApiError>

Verifies a JWT whose iss is a DID: signed by a key the DID logs in with, for this server, for lxm, unexpired and unspent. The DID must be hosted here, and its keys are its credential records.