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/pallet-xcm/Cargo.toml

@@ -23,12 +23,12 @@ sp-std = { path = "../../../substrate/primitives/std", default-features = false}
 
 xcm = { package = "staging-xcm", path = "..", default-features = false }
 xcm-executor = { package = "staging-xcm-executor", path = "../xcm-executor", default-features = false }
+xcm-builder = { package = "staging-xcm-builder", path = "../xcm-builder", default-features = false }
 
 [dev-dependencies]
 pallet-balances = { path = "../../../substrate/frame/balances" }
 polkadot-runtime-parachains = { path = "../../runtime/parachains" }
 polkadot-parachain-primitives = { path = "../../parachain" }
-xcm-builder = { package = "staging-xcm-builder", path = "../xcm-builder" }
 
 [features]
 default = [ "std" ]
@@ -45,6 +45,7 @@ std = [
 	"sp-io/std",
 	"sp-runtime/std",
 	"sp-std/std",
+	"xcm-builder/std",
 	"xcm-executor/std",
 	"xcm/std",
 ]