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.

---------

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>
Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>
Co-authored-by: command-bot <>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Alexander Theißen <alex.theissen@me.com>

substrate/frame/contracts/src/tests.rs

@@ -486,6 +486,7 @@ impl Config for Test {
 	type MaxDelegateDependencies = MaxDelegateDependencies;
 	type Debug = TestDebug;
 	type Environment = ();
+	type Xcm = ();
 }
 
 pub const ALICE: AccountId32 = AccountId32::new([1u8; 32]);