Function signature_is_valid_core 

async fn signature_is_valid_core(
    method: &str,
    provided_signature: &str,
    region: &str,
    service: &str,
    datetime: DateTime<Utc>,
    full_url: &str,
    headers: &HeaderMap,
    payload_override: Option<String>,
    access_key: &str,
    secret_key: &str,
    signed_headers: &Vec<String>,
    body_bytes: &[u8],
) -> Result<bool, Box<dyn Error>>

Core signature validation: compares provided vs computed