Coretime Feature branch (relay chain) (#1694)

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

---------

Co-authored-by: eskimor <eskimor@no-such-url.com>
Co-authored-by: Bradley Olson <34992650+BradleyOlson64@users.noreply.github.com>
Co-authored-by: BradleyOlson64 <lotrftw9@gmail.com>
Co-authored-by: Anton Vilhelm Ásgeirsson <antonva@users.noreply.github.com>
Co-authored-by: antonva <anton.asgeirsson@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Marcin S. <marcin@realemail.net>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: command-bot <>

Cargo.lock

@@ -12993,6 +12993,7 @@ dependencies = [
  "pallet-authorship",
  "pallet-babe",
  "pallet-balances",
+ "pallet-broker",
  "pallet-election-provider-multi-phase",
  "pallet-fast-unstake",
  "pallet-identity",
@@ -13063,6 +13064,7 @@ dependencies = [
  "pallet-authorship",
  "pallet-babe",
  "pallet-balances",
+ "pallet-broker",
  "pallet-message-queue",
  "pallet-session",
  "pallet-staking",
@@ -13083,6 +13085,7 @@ dependencies = [
  "serde_json",
  "sp-api",
  "sp-application-crypto",
+ "sp-arithmetic",
  "sp-core",
  "sp-inherents",
  "sp-io",