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/mock-network/src/lib.rs

@@ -26,7 +26,8 @@ use crate::primitives::{AccountId, UNITS};
 use sp_runtime::BuildStorage;
 use xcm::latest::prelude::*;
 use xcm_executor::traits::ConvertLocation;
-use xcm_simulator::{decl_test_network, decl_test_parachain, decl_test_relay_chain, TestExt};
+pub use xcm_simulator::TestExt;
+use xcm_simulator::{decl_test_network, decl_test_parachain, decl_test_relay_chain};
 
 // Accounts
 pub const ADMIN: sp_runtime::AccountId32 = sp_runtime::AccountId32::new([0u8; 32]);