Function proof 

pub fn proof(
    key: &SigningKey,
    method: &str,
    url: &str,
    access_token: Option<&str>,
    nonce: Option<&str>,
) -> Result<String>

Build a DPoP proof for one request.

access_token binds the proof to that token via ath; pass it for every resource request. Without it a captured proof can be replayed alongside a different token.

nonce is the value from a previous DPoP-Nonce response header, supplied on the retry after a nonce_challenge.