Add feature flag to enable v2 assignments (#2444)

Scaffold everything, so that we can enable v2 assignments via a node
feature bit, once all nodes have upgraded to the new protocol.

Implements: https://github.com/paritytech/polkadot-sdk/issues/628

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

polkadot/node/network/availability-distribution/src/tests/state.rs

@@ -46,8 +46,8 @@ use polkadot_node_subsystem::{
 };
 use polkadot_node_subsystem_test_helpers as test_helpers;
 use polkadot_primitives::{
-	CandidateHash, CoreState, ExecutorParams, GroupIndex, Hash, Id as ParaId, ScheduledCore,
-	SessionInfo, ValidatorIndex,
+	vstaging::NodeFeatures, CandidateHash, CoreState, ExecutorParams, GroupIndex, Hash,
+	Id as ParaId, ScheduledCore, SessionInfo, ValidatorIndex,
 };
 use test_helpers::mock::{make_ferdie_keystore, new_leaf};
 
@@ -264,6 +264,9 @@ impl TestState {
 							tx.send(Ok(Some(ExecutorParams::default())))
 								.expect("Receiver should be alive.");
 						},
+						RuntimeApiRequest::NodeFeatures(_, si_tx) => {
+							si_tx.send(Ok(NodeFeatures::EMPTY)).expect("Receiver should be alive.");
+						},
 						RuntimeApiRequest::AvailabilityCores(tx) => {
 							gum::trace!(target: LOG_TARGET, cores= ?self.cores[&hash], hash = ?hash, "Sending out cores for hash");
 							tx.send(Ok(self.cores[&hash].clone()))