Function fetch_release 

pub fn fetch_release(
    source: &dyn ReleaseSource,
    verifier: &dyn AttestationVerifier,
    tag: &str,
    progress: &mut dyn FnMut(&str),
) -> Result<Fetched, String>

Release tag’s ASSET, proven by attestation or, when no answer about its bundle can be had, by checksum. See the module docs for the order and why it’s that order.