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/primitives/src/vstaging/mod.rs

@@ -22,3 +22,19 @@ use bitvec::vec::BitVec;
 
 /// Bit indices in the `HostConfiguration.node_features` that correspond to different node features.
 pub type NodeFeatures = BitVec<u8, bitvec::order::Lsb0>;
+
+/// Module containing feature-specific bit indices into the `NodeFeatures` bitvec.
+pub mod node_features {
+	/// A feature index used to indentify a bit into the node_features array stored
+	/// in the HostConfiguration.
+	#[repr(u8)]
+	pub enum FeatureIndex {
+		/// Tells if tranch0 assignments could be sent in a single certificate.
+		/// Reserved for: `<https://github.com/paritytech/polkadot-sdk/issues/628>`
+		EnableAssignmentsV2 = 0,
+		/// First unassigned feature bit.
+		/// Every time a new feature flag is assigned it should take this value.
+		/// and this should be incremented.
+		FirstUnassigned = 1,
+	}
+}