A pallet which uses the XCMP transport layer to handle both incoming and outgoing XCM message
sending and dispatch, queuing, signalling and backpressure. To do so, it implements:
* `XcmpMessageHandler`
* `XcmpMessageSource`
Also provides an implementation of `SendXcm` which can be placed in a router tuple for relaying
XCM over XCMP if the destination is `Parent/Parachain`. It requires an implementation of
`XcmExecutor` for dispatching incoming XCM messages.