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/riscv32.rs

@@ -297,7 +297,7 @@ impl HostFn for HostFnImpl {
 		todo!()
 	}
 
-	fn xcm_execute(msg: &[u8], output: &mut &mut [u8]) -> Result {
+	fn xcm_execute(msg: &[u8]) -> Result {
 		todo!()
 	}
 

substrate/frame/contracts/uapi/src/host/wasm32.rs

@@ -160,7 +160,7 @@ mod sys {
 
 		pub fn weight_to_fee(gas: u64, output_ptr: *mut u8, output_len_ptr: *mut u32);
 
-		pub fn xcm_execute(msg_ptr: *const u8, msg_len: u32, output_ptr: *mut u8) -> ReturnCode;
+		pub fn xcm_execute(msg_ptr: *const u8, msg_len: u32) -> ReturnCode;
 
 		pub fn xcm_send(
 			dest_ptr: *const u8,
@@ -819,9 +819,8 @@ impl HostFn for HostFnImpl {
 		unsafe { sys::reentrance_count() }
 	}
 
-	fn xcm_execute(msg: &[u8], output: &mut &mut [u8]) -> Result {
-		let ret_code =
-			unsafe { sys::xcm_execute(msg.as_ptr(), msg.len() as _, output.as_mut_ptr()) };
+	fn xcm_execute(msg: &[u8]) -> Result {
+		let ret_code = unsafe { sys::xcm_execute(msg.as_ptr(), msg.len() as _) };
 		ret_code.into()
 	}