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/core/approval-voting/src/tests.rs

@@ -37,8 +37,8 @@ use polkadot_node_subsystem_test_helpers as test_helpers;
 use polkadot_node_subsystem_util::TimeoutExt;
 use polkadot_overseer::HeadSupportsParachains;
 use polkadot_primitives::{
-	CandidateCommitments, CandidateEvent, CoreIndex, GroupIndex, Header, Id as ParaId, IndexedVec,
-	ValidationCode, ValidatorSignature,
+	vstaging::NodeFeatures, CandidateCommitments, CandidateEvent, CoreIndex, GroupIndex, Header,
+	Id as ParaId, IndexedVec, ValidationCode, ValidatorSignature,
 };
 use std::time::Duration;
 
@@ -243,6 +243,7 @@ where
 			polkadot_primitives::CoreIndex,
 			polkadot_primitives::GroupIndex,
 		)>,
+		_enable_assignments_v2: bool,
 	) -> HashMap<polkadot_primitives::CoreIndex, criteria::OurAssignment> {
 		self.0()
 	}
@@ -1003,6 +1004,15 @@ async fn import_block(
 					si_tx.send(Ok(Some(ExecutorParams::default()))).unwrap();
 				}
 			);
+
+			assert_matches!(
+				overseer_recv(overseer).await,
+				AllMessages::RuntimeApi(
+					RuntimeApiMessage::Request(_, RuntimeApiRequest::NodeFeatures(_, si_tx), )
+				) => {
+					si_tx.send(Ok(NodeFeatures::EMPTY)).unwrap();
+				}
+			);
 		}
 
 		assert_matches!(