elastic scaling: add core selector to cumulus (#5372)

Partially implements
https://github.com/paritytech/polkadot-sdk/issues/5048

- adds a core selection runtime API to cumulus and a generic way of
configuring it for a parachain
- modifies the slot based collator to utilise the claim queue and the
generic core selection

What's left to be implemented (in a follow-up PR):
- add the UMP signal for core selection into the parachain-system pallet

View the RFC for more context:
https://github.com/polkadot-fellows/RFCs/pull/103

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Co-authored-by: command-bot <>