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>