Contracts: Add XCM traits to interface with contracts (#2086)

We are introducing a new set of `XcmController` traits (final name yet
to be determined).
These traits are implemented by `pallet-xcm` and allows other pallets,
such as `pallet_contracts`, to rely on these traits instead of tight
coupling them to `pallet-xcm`.

Using only the existing Xcm traits would mean duplicating the logic from
`pallet-xcm` in these other pallets, which we aim to avoid. Our
objective is to ensure that when these APIs are called from
`pallet-contracts`, they produce the exact same outcomes as if called
directly from `pallet-xcm`.

The other benefits is that we can also expose return values to
`pallet-contracts` instead of just calling `pallet-xcm` dispatchable and
getting a `DispatchResult` back.

See traits integration in this PR
https://github.com/paritytech/polkadot-sdk/pull/1248, where the traits
are used as follow to define and implement `pallet-contracts` Config.
```rs
// Contracts config:
pub trait Config: frame_system::Config {
  // ...

  /// A type that exposes XCM APIs, allowing contracts to interact with other parachains, and
  /// execute XCM programs.
  type Xcm: xcm_executor::traits::Controller<
	  OriginFor<Self>,
	  <Self as frame_system::Config>::RuntimeCall,
	  BlockNumberFor<Self>,
  >;
}

// implementation
impl pallet_contracts::Config for Runtime {
        // ...

	type Xcm = pallet_xcm::Pallet<Self>;
}
```

---------

Co-authored-by: Alexander Theißen <alex.theissen@me.com>
Co-authored-by: command-bot <>

polkadot/xcm/xcm-builder/src/lib.rs

@@ -115,3 +115,9 @@ pub use origin_aliases::AliasForeignAccountId32;
 
 mod pay;
 pub use pay::{FixedLocation, LocatableAssetId, PayAccountId32OnChainOverXcm, PayOverXcm};
+
+mod controller;
+pub use controller::{
+	Controller, ExecuteController, ExecuteControllerWeightInfo, QueryController,
+	QueryControllerWeightInfo, SendController, SendControllerWeightInfo,
+};