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/fixtures/src/lib.rs

@@ -23,6 +23,8 @@ fn fixtures_root_dir() -> PathBuf {
 		// When `CARGO_MANIFEST_DIR` is not set, Rust resolves relative paths from the root folder
 		(Err(_), _) => "substrate/frame/contracts/fixtures/data".into(),
 		(Ok(path), Ok(s)) if s == "pallet-contracts" => PathBuf::from(path).join("fixtures/data"),
+		(Ok(path), Ok(s)) if s == "pallet-contracts-mock-network" =>
+			PathBuf::from(path).parent().unwrap().join("fixtures/data"),
 		(Ok(_), pkg_name) => panic!("Failed to resolve fixture dir for tests from {pkg_name:?}."),
 	}
 }