Contracts expose pallet-xcm (#1248)

This PR introduces:
- XCM  host functions `xcm_send`, `xcm_execute`
- An Xcm trait into the config. that proxy these functions to to
`pallet_xcm`, or disable their usage by using `()`.
- A mock_network and xcm_test files to test the newly added xcm-related
functions.

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Co-authored-by: Keith Yeung <[email protected]>
Co-authored-by: Sasha Gryaznov <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: Alexander Theißen <[email protected]>