Also fixes: https://github.com/paritytech/polkadot-sdk/issues/1417

- [x] CoreIndex -> AssignmentProvider mapping will be able to change any
time.
- [x] Implement
- [x] Provide Migrations
- [x] Add and fix tests
- [x] Implement bulk assigner logic
- [x] bulk assigner tests
- [x] Port over current assigner to use bulk designer (+ share on-demand
with bulk): top-level assigner has core ranges: legacy, bulk
- [x] Adjust migrations to reflect new assigner structure
- [x] Move migration code to Assignment code directly and make it
recursive (make it possible to skip releases) -> follow up ticket.
- [x] Test migrations
- [x] Add migration PR to runtimes repo -> follow up ticket.
- [x] Wire up with actual UMP messages
- [x] Write PR docs

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Co-authored-by: eskimor <[email protected]>
Co-authored-by: Bradley Olson <[email protected]>
Co-authored-by: BradleyOlson64 <[email protected]>
Co-authored-by: Anton Vilhelm Ásgeirsson <[email protected]>
Co-authored-by: antonva <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Marcin S. <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>