Runtime: allow backing multiple candidates of same parachain on different cores (#3231)

Fixes https://github.com/paritytech/polkadot-sdk/issues/3144

Builds on top of https://github.com/paritytech/polkadot-sdk/pull/3229

### Summary
Some preparations for Runtime to support elastic scaling, guarded by
config node features bit `FeatureIndex::ElasticScalingMVP`. This PR
introduces a per-candidate `CoreIndex` but does it in a hacky way to
avoid changing `CandidateCommitments`, `CandidateReceipts` primitives
and networking protocols.

#### Including `CoreIndex` in `BackedCandidate`
If the `ElasticScalingMVP` feature bit is enabled then
`BackedCandidate::validator_indices` is extended by 8 bits.
The value stored in these bits represents the assumed core index for the
candidate.

It is temporary solution which works by creating a mapping from
`BackedCandidate` to `CoreIndex` by assuming the `CoreIndex` can be
discovered by checking in which validator group the validator that
signed the statement is.

TODO:
- [x] fix tests
- [x] add new tests
- [x] Bump runtime API for Kusama, so we have that node features thing!
-> https://github.com/polkadot-fellows/runtimes/pull/194

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Signed-off-by: alindima <alin@parity.io>
Co-authored-by: alindima <alin@parity.io>

polkadot/node/core/provisioner/src/tests.rs

@@ -460,13 +460,16 @@ mod select_candidates {
 
 		let expected_backed = expected_candidates
 			.iter()
-			.map(|c| BackedCandidate {
-				candidate: CommittedCandidateReceipt {
-					descriptor: c.descriptor.clone(),
-					commitments: Default::default(),
-				},
-				validity_votes: Vec::new(),
-				validator_indices: default_bitvec(MOCK_GROUP_SIZE),
+			.map(|c| {
+				BackedCandidate::new(
+					CommittedCandidateReceipt {
+						descriptor: c.descriptor().clone(),
+						commitments: Default::default(),
+					},
+					Vec::new(),
+					default_bitvec(MOCK_GROUP_SIZE),
+					None,
+				)
 			})
 			.collect();
 
@@ -486,7 +489,7 @@ mod select_candidates {
 
 				result.into_iter().for_each(|c| {
 					assert!(
-						expected_candidates.iter().any(|c2| c.candidate.corresponds_to(c2)),
+						expected_candidates.iter().any(|c2| c.candidate().corresponds_to(c2)),
 						"Failed to find candidate: {:?}",
 						c,
 					)
@@ -532,10 +535,13 @@ mod select_candidates {
 		// Build possible outputs from select_candidates
 		let backed_candidates: Vec<_> = committed_receipts
 			.iter()
-			.map(|committed_receipt| BackedCandidate {
-				candidate: committed_receipt.clone(),
-				validity_votes: Vec::new(),
-				validator_indices: default_bitvec(MOCK_GROUP_SIZE),
+			.map(|committed_receipt| {
+				BackedCandidate::new(
+					committed_receipt.clone(),
+					Vec::new(),
+					default_bitvec(MOCK_GROUP_SIZE),
+					None,
+				)
 			})
 			.collect();
 
@@ -566,7 +572,7 @@ mod select_candidates {
 
 				result.into_iter().for_each(|c| {
 					assert!(
-						expected_backed_filtered.iter().any(|c2| c.candidate.corresponds_to(c2)),
+						expected_backed_filtered.iter().any(|c2| c.candidate().corresponds_to(c2)),
 						"Failed to find candidate: {:?}",
 						c,
 					)
@@ -605,13 +611,16 @@ mod select_candidates {
 
 		let expected_backed = expected_candidates
 			.iter()
-			.map(|c| BackedCandidate {
-				candidate: CommittedCandidateReceipt {
-					descriptor: c.descriptor.clone(),
-					commitments: Default::default(),
-				},
-				validity_votes: Vec::new(),
-				validator_indices: default_bitvec(MOCK_GROUP_SIZE),
+			.map(|c| {
+				BackedCandidate::new(
+					CommittedCandidateReceipt {
+						descriptor: c.descriptor.clone(),
+						commitments: Default::default(),
+					},
+					Vec::new(),
+					default_bitvec(MOCK_GROUP_SIZE),
+					None,
+				)
 			})
 			.collect();
 
@@ -631,7 +640,7 @@ mod select_candidates {
 
 				result.into_iter().for_each(|c| {
 					assert!(
-						expected_candidates.iter().any(|c2| c.candidate.corresponds_to(c2)),
+						expected_candidates.iter().any(|c2| c.candidate().corresponds_to(c2)),
 						"Failed to find candidate: {:?}",
 						c,
 					)
@@ -671,13 +680,16 @@ mod select_candidates {
 
 		let expected_backed = expected_candidates
 			.iter()
-			.map(|c| BackedCandidate {
-				candidate: CommittedCandidateReceipt {
-					descriptor: c.descriptor.clone(),
-					commitments: Default::default(),
-				},
-				validity_votes: Vec::new(),
-				validator_indices: default_bitvec(MOCK_GROUP_SIZE),
+			.map(|c| {
+				BackedCandidate::new(
+					CommittedCandidateReceipt {
+						descriptor: c.descriptor().clone(),
+						commitments: Default::default(),
+					},
+					Vec::new(),
+					default_bitvec(MOCK_GROUP_SIZE),
+					None,
+				)
 			})
 			.collect();
 
@@ -697,7 +709,7 @@ mod select_candidates {
 
 				result.into_iter().for_each(|c| {
 					assert!(
-						expected_candidates.iter().any(|c2| c.candidate.corresponds_to(c2)),
+						expected_candidates.iter().any(|c2| c.candidate().corresponds_to(c2)),
 						"Failed to find candidate: {:?}",
 						c,
 					)