This PR notifies broker pallet for any parachain slot swaps performed on
the relay chain. This is achieved by registering an `OnSwap` for the the
`coretime` pallet. The hook sends XCM message to the broker chain and
invokes a new extrinsic `swap_leases` which updates `Leases` storage
item (which keeps the legacy parachain leases).

I made two assumptions in this PR:
1.
[`Leases`](https://github.com/paritytech/polkadot-sdk/blob/4987d798/substrate/frame/broker/src/lib.rs#L120)
in `broker` pallet and
[`Leases`](https://github.com/paritytech/polkadot-sdk/blob/4987d798

/polkadot/runtime/common/src/slots/mod.rs#L118)
in `slots` pallet are in sync.
2. `swap_leases` extrinsic from `broker` pallet can be triggered only by
root or by the XCM message from the relay chain. If not - the extrinsic
will generate an error and do nothing.

As a side effect from the changes `OnSwap` trait is moved from
runtime/common/traits.rs to runtime/parachains. Otherwise it is not
accessible from `broker` pallet.

Closes https://github.com/paritytech/polkadot-sdk/issues/3552

TODOs:

- [x] Weights
- [x] Tests

---------

Co-authored-by: command-bot <>
Co-authored-by: eskimor <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>