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/lib.rs

@@ -403,6 +403,14 @@ pub mod pallet {
 		/// its type appears in the metadata. Only valid value is `()`.
 		#[pallet::constant]
 		type Environment: Get<Environment<Self>>;
+
+		/// A type that exposes XCM APIs, allowing contracts to interact with other parachains, and
+		/// execute XCM programs.
+		type Xcm: xcm_builder::Controller<
+			OriginFor<Self>,
+			<Self as frame_system::Config>::RuntimeCall,
+			BlockNumberFor<Self>,
+		>;
 	}
 
 	#[pallet::hooks]
@@ -1004,6 +1012,8 @@ pub mod pallet {
 		/// in this error. Note that this usually  shouldn't happen as deploying such contracts
 		/// is rejected.
 		NoChainExtension,
+		/// Failed to decode the XCM program.
+		XCMDecodeFailed,
 		/// A contract with the same AccountId already exists.
 		DuplicateContract,
 		/// A contract self destructed in its constructor.