Implements most of
https://github.com/paritytech/polkadot-sdk/issues/1797

Core sharing (two parachains or more marachains scheduled on the same
core with the same `PartsOf57600` value) was not working correctly. The
expected behaviour is to have Backed and Included event in each block
for the paras sharing the core and the paras should take turns. E.g. for
two cores we expect: Backed(a); Included(a)+Backed(b);
Included(b)+Backed(a); etc. Instead of this each block contains just one
event and there are a lot of gaps (blocks w/o events) during the
session.

Core sharing should also work when collators are building collations
ahead of time

TODOs:

- [x] Add a zombienet test verifying that the behaviour mentioned above
works.
- [x] prdoc

---------

Co-authored-by: alindima <alin@parity.io>

One more try to make this test robust from a resource perspective.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>

On top of https://github.com/paritytech/polkadot-sdk/pull/3879

I've also moved the previous test where we ensure multiple cores per
para doesn't break non elastic parachains.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Javier Viola <363911+pepoviola@users.noreply.github.com>

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>