pvf-precheck: PVF pre-checker subsystem (#4643)

This commit implements the last major piece of #3211: the subsystem that
tracks PVFs that require voting, issues pre-check requests to
candidate-validation and makes sure that the votes are submitted to the
chain.

polkadot/node/subsystem-types/src/messages.rs

@@ -910,3 +910,9 @@ pub enum GossipSupportMessage {
 	#[from]
 	NetworkBridgeUpdateV1(NetworkBridgeEvent<protocol_v1::GossipSuppportNetworkMessage>),
 }
+
+/// PVF checker message.
+///
+/// Currently non-instantiable.
+#[derive(Debug)]
+pub enum PvfCheckerMessage {}