client/authority-discovery: Remove sentry node logic (#7368)

* client/authority-discovery: Remove sentry node logic

The notion of sentry nodes has been deprecated (see [1] for details).
This commit removes support for sentry nodes in the
`client/authority-discovery` module.

While removing `Role::Sentry` this commit also introduces
`Role::Discover`, allowing a node to discover addresses of authorities
without publishing ones own addresses. This will be needed in Polkadot
for collator nodes.

[1] https://github.com/paritytech/substrate/issues/6845

* client/authority-discovery/service: Improve PeerId comment

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

@@ -55,9 +55,8 @@ fn get_addresses_and_authority_id() {
 	let (mut worker, mut service) = new_worker_and_service(
 		test_api,
 		network.clone(),
-		vec![],
 		Box::pin(dht_event_rx),
-		Role::Authority(key_store.into()),
+		Role::PublishAndDiscover(key_store.into()),
 		None,
 	);
 	worker.inject_addresses(remote_authority_id.clone(), vec![remote_addr.clone()]);