Elastic scaling: use an assumed CoreIndex in candidate-backing (#3229)

First step in implementing
https://github.com/paritytech/polkadot-sdk/issues/3144

### Summary of changes
- switch statement `Table` candidate mapping from `ParaId` to
`CoreIndex`
- introduce experimental `InjectCoreIndex`  node feature.
- determine and assume a `CoreIndex` for a candidate based on statement
validator index. If the signature is valid it means validator controls
the validator that index and we can easily map it to a validator
group/core.
- introduce a temporary provisioner fix until we fully enable elastic
scaling in the subystem. The fix ensures we don't fetch the same
backable candidate when calling `get_backable_candidate` for each core.

TODO:
- [x] fix backing tests
- [x] fix statement table tests
- [x] add new test

---------

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/lib.rs

@@ -681,10 +681,17 @@ async fn request_backable_candidates(
 			CoreState::Free => continue,
 		};
 
+		// We should be calling this once per para rather than per core.
+		// TODO: Will be fixed in https://github.com/paritytech/polkadot-sdk/pull/3233.
+		// For now, at least make sure we don't supply the same candidate multiple times in case a
+		// para has multiple cores scheduled.
 		let response = get_backable_candidate(relay_parent, para_id, required_path, sender).await?;
-
 		match response {
-			Some((hash, relay_parent)) => selected_candidates.push((hash, relay_parent)),
+			Some((hash, relay_parent)) => {
+				if !selected_candidates.iter().any(|bc| &(hash, relay_parent) == bc) {
+					selected_candidates.push((hash, relay_parent))
+				}
+			},
 			None => {
 				gum::debug!(
 					target: LOG_TARGET,
@@ -726,6 +733,7 @@ async fn select_candidates(
 			)
 			.await?,
 	};
+	gum::debug!(target: LOG_TARGET, ?selected_candidates, "Got backable candidates");
 
 	// now get the backed candidates corresponding to these candidate receipts
 	let (tx, rx) = oneshot::channel();