Support sending and receiving multiple Justifications (#8266)

* client/network: support sending multiple justifications

* network: flag support for multiple justifications in protobuf request

* Update client/network/src/protocol.rs

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* network: update comment on protobuf field

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

substrate/client/network/src/protocol/sync/blocks.rs

@@ -228,6 +228,7 @@ mod test {
 			message_queue: None,
 			receipt: None,
 			justification: None,
+			justifications: None,
 		}).collect()
 	}