Elastic scaling: runtime dependency tracking and enactment (#3479)

Changes needed to implement the runtime part of elastic scaling:
https://github.com/paritytech/polkadot-sdk/issues/3131,
https://github.com/paritytech/polkadot-sdk/issues/3132,
https://github.com/paritytech/polkadot-sdk/issues/3202

Also fixes https://github.com/paritytech/polkadot-sdk/issues/3675

TODOs:

- [x] storage migration
- [x] optimise process_candidates from O(N^2)
- [x] drop backable candidates which form cycles
- [x] fix unit tests
- [x] add more unit tests
- [x] check the runtime APIs which use the pending availability storage.
We need to expose all of them, see
https://github.com/paritytech/polkadot-sdk/issues/3576
- [x] optimise the candidate selection. we're currently picking randomly
until we satisfy the weight limit. we need to be smart about not
breaking candidate chains while being fair to all paras -
https://github.com/paritytech/polkadot-sdk/pull/3573

Relies on the changes made in
https://github.com/paritytech/polkadot-sdk/pull/3233 in terms of the
inclusion policy and the candidate ordering

---------

Signed-off-by: alindima <alin@parity.io>
Co-authored-by: command-bot <>
Co-authored-by: eskimor <eskimor@users.noreply.github.com>

polkadot/roadmap/implementers-guide/src/runtime/scheduler.md

@@ -285,7 +285,6 @@ No finalization routine runs for this module.
   - This clears them from `Scheduled` and marks each corresponding `core` in the `AvailabilityCores` as occupied.
   - Since both the availability cores and the newly-occupied cores lists are sorted ascending, this method can be
     implemented efficiently.
-- `core_para(CoreIndex) -> ParaId`: return the currently-scheduled or occupied ParaId for the given core.
 - `group_validators(GroupIndex) -> Option<Vec<ValidatorIndex>>`: return all validators in a given group, if the group
   index is valid for this session.
 - `availability_timeout_predicate() -> Option<impl Fn(CoreIndex, BlockNumber) -> bool>`: returns an optional predicate