Function authorization_header 

pub fn authorization_header(
    method: &str,
    url: &SignedUrl,
    creds: &SigV4Credentials<'_>,
    amz_date: &str,
    content_type: Option<&str>,
    payload_hash: &str,
) -> String

Build the Authorization header value for a request.

amz_date is YYYYMMDDTHHMMSSZ; payload_hash is the hex SHA-256 of the exact body bytes sent (GET uses the empty-body hash).