Contracts: xcm host fn fixes (#3086)

## Xcm changes:
- Fix `pallet_xcm::execute`, move the logic into The `ExecuteController`
so it can be shared with anything that implement that trait.
- Make `ExecuteController::execute` retursn `DispatchErrorWithPostInfo`
instead of `DispatchError`, so that we don't charge the full
`max_weight` provided if the execution is incomplete (useful for
force_batch or contracts calls)
- Fix docstring for `pallet_xcm::execute`, to reflect the changes from
#2405
- Update the signature for `ExecuteController::execute`, we don't need
to return the `Outcome` anymore since we only care about
`Outcome::Complete`

## Contracts changes:

- Update host fn `xcm_exexute`, we don't need to write the `Outcome` to
the sandbox memory anymore. This was also not charged as well before so
it if fixes this too.
- One of the issue was that the dry_run of a contract that call
`xcm_execute` would exhaust the `gas_limit`.

This is because `XcmExecuteController::execute` takes a `max_weight`
argument, and since we don't want the user to specify it manually we
were passing everything left by pre-charghing
`ctx.ext.gas_meter().gas_left()`

- To fix it I added a `fn influence_lowest_limit` on the `Token` trait
and make it return false for `RuntimeCost::XcmExecute`.
- Got rid of the `RuntimeToken` indirection, we can just use
`RuntimeCost` directly.

---------

Co-authored-by: command-bot <>

substrate/frame/contracts/uapi/src/host.rs

@@ -792,7 +792,7 @@ pub trait HostFn {
 	#[deprecated(
 		note = "Unstable function. Behaviour can change without further notice. Use only for testing."
 	)]
-	fn xcm_execute(msg: &[u8], output: &mut &mut [u8]) -> Result;
+	fn xcm_execute(msg: &[u8]) -> Result;
 
 	/// Send an XCM program from the contract to the specified destination.
 	/// This is equivalent to dispatching `pallet_xcm::send` through `call_runtime`, except that
@@ -804,7 +804,6 @@ pub trait HostFn {
 	///   traps otherwise.
 	/// - `msg`: The message, should be decodable as a [VersionedXcm](https://paritytech.github.io/polkadot-sdk/master/staging_xcm/enum.VersionedXcm.html),
 	///   traps otherwise.
-	/// - `output`: A reference to the output data buffer to write the [XcmHash](https://paritytech.github.io/polkadot-sdk/master/staging_xcm/v3/type.XcmHash.html)
 	///
 	/// # Return
 	///