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/runtime/parachains/src/builder.rs

@@ -587,11 +587,12 @@ impl<T: paras_inherent::Config> BenchBuilder<T> {
 					})
 					.collect();
 
-				BackedCandidate::<T::Hash> {
+				BackedCandidate::<T::Hash>::new(
 					candidate,
 					validity_votes,
-					validator_indices: bitvec::bitvec![u8, bitvec::order::Lsb0; 1; group_validators.len()],
-				}
+					bitvec::bitvec![u8, bitvec::order::Lsb0; 1; group_validators.len()],
+					None,
+				)
 			})
 			.collect()
 	}