Track nested memory used by xcm::DoubleEncoded

[serban300]

Resolves #8675

The PR introduces a global variable that limits the amount of heap memory available for decoding structures within nested contexts.

[bkchr]

I find the current approach hard to follow and it also doesn't forward the left over limit from decoding the extrinsic.

E.g. when executing pallet_xcm::execute it should only leave the rest from the budget for Transact and not give it the full budget again. Also the budgeting right now maybe shares too much. E.g. when I want to load two XCM messages from the storage, they will both use the same limit. This is probably not wanted. We should provide some better "tools" around the bounded limit.

struct DecodeWithLimit {
       limit: usize,
       decoded: T,
}

impl Decode using DecodeWithLimitThingy {}

fn DecodeWithLimit {
     pub use(use: Impl(&mut T) -> R) -> R {
              mem::run_with(self.limit, || use(&mut self.decoded))
     }
}

Maybe something like this? Then we could use this to load XCM messages from storage?

[bkchr]

If we want that to be generic, we should pass the limit as an input value and not use this constant.

[bkchr]

The problem here is that we will run here without knowing how much the outer extrinsic already used for decoding.

[bkchr]

The scope is way too broad here. We only need this for extrinsic applying.