Put all authorities of a session into SessionInfo. (#3813)

* Make SessionInfo include all authorities

of the current session.

* Add missing import.

* Only take subset for assignment keys.

* Fix typo.

* Make spellcheck happy.

* Really.

* Fix test.

* More clear documentation.

* Update comments in `SessionInfo`.

polkadot/node/subsystem-util/src/runtime/mod.rs

@@ -199,7 +199,7 @@ impl RuntimeInfo {
 	/// Build `ValidatorInfo` for the current session.
 	///
 	///
-	/// Returns: `None` if not a validator.
+	/// Returns: `None` if not a parachain validator.
 	async fn get_validator_info(&self, session_info: &SessionInfo) -> Result<ValidatorInfo> {
 		if let Some(our_index) = self.get_our_index(&session_info.validators).await {
 			// Get our group index: