Add broker pallet to `coretime-westend` (#3272)

This brings functionality to Westend's Coretime Chain runtime, where
previously it was not much more than a shell.

It is assumed that the Coretime pallet will have the same index in the
Westend runtime as it does in Rococo for the runtime calls.

TODO:
- [x] Generate chainspec
- [x] Regenerate weights
- [x] Check hardcoded RuntimeCall weights against relay weights for
transacts

Aura key generation: https://github.com/paritytech/devops/issues/2725

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Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Anton Vilhelm Ásgeirsson <antonva@users.noreply.github.com>