Remove HeaderBackend requirement from AuthorityDiscovery and NetworkWorker (#13730)

* Remove `HeaderBackend` requirement from `NetworkWorker`

* Remove HeaderBackend from authority-discovery

substrate/client/authority-discovery/src/error.rs

@@ -23,6 +23,7 @@ pub type Result<T> = std::result::Result<T, Error>;
 
 /// Error type for the authority discovery module.
 #[derive(Debug, thiserror::Error)]
+#[allow(missing_docs)]
 pub enum Error {
 	#[error("Received dht value found event with records with different keys.")]
 	ReceivingDhtValueFoundEventWithDifferentKeys,
@@ -71,4 +72,7 @@ pub enum Error {
 
 	#[error("Received authority record without a valid signature for the remote peer id.")]
 	MissingPeerIdSignature,
+
+	#[error("Unable to fetch best block.")]
+	BestBlockFetchingError,
 }