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/fixtures/contracts/xcm_execute.rs

@@ -30,10 +30,11 @@ pub extern "C" fn deploy() {}
 pub extern "C" fn call() {
 	input!(512, msg: [u8],);
 
-	let mut outcome = [0u8; 512];
-	let outcome = &mut &mut outcome[..];
-
 	#[allow(deprecated)]
-	api::xcm_execute(msg, outcome).unwrap();
-	api::return_value(uapi::ReturnFlags::empty(), outcome);
+	let err_code = match api::xcm_execute(msg) {
+		Ok(_) => 0u32,
+		Err(code) => code as u32,
+	};
+
+	api::return_value(uapi::ReturnFlags::empty(), &err_code.to_le_bytes());
 }