Function open_admin_credential 

pub fn open_admin_credential(
    bundle_path: &Path,
    config_dir: &Path,
    expect_digest: Option<&str>,
    expected_vta_did: Option<&str>,
) -> Result<CredentialBundle, Box<dyn Error>>

Open an armored admin-credential bundle and verify where it came from before anything is installed.

Opens the bundle like open_armored_bundle, then applies verify_admin_bundle. expect_digest of None is the --no-verify-digest case: only a bundle DidSigned by expected_vta_did is then accepted.

The single-use request secret is removed only once the bundle has been accepted, so a rejected bundle does not use up the pending request.