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/zombienet_tests/functional/0012-elastic-scaling-mvp.toml

@@ -0,0 +1,38 @@
+[settings]
+timeout = 1000
+bootnode = true
+
+[relaychain.genesis.runtimeGenesis.patch.configuration.config]
+  max_validators_per_core = 2
+  needed_approvals = 4
+  coretime_cores = 2
+
+[relaychain]
+default_image = "{{ZOMBIENET_INTEGRATION_TEST_IMAGE}}"
+chain = "rococo-local"
+default_command = "polkadot"
+
+[relaychain.default_resources]
+limits = { memory = "4G", cpu = "2" }
+requests = { memory = "2G", cpu = "1" }
+
+  [[relaychain.nodes]]
+  name = "alice"
+  validator = "true"
+
+  [[relaychain.node_groups]]
+  name = "validator"
+  count = 3
+  args = [ "-lparachain=debug,runtime=debug"]
+
+[[parachains]]
+id = 2000
+default_command = "polkadot-parachain"
+add_to_genesis = false
+register_para = true
+onboard_as_parachain = false
+
+  [parachains.collator]
+  name = "collator2000"
+  command = "polkadot-parachain"
+  args = [ "-lparachain=debug" ]