fix #5574

- Use U256 instead of BalanceOf<T> and MomentOf<T> in Ext trait
- Enforce H256 for T::Hash

The Ext trait still depends on the associated type `T: Config`, we can
look into refactoring it even more later but even in the current state
it should not influence how the data is encoded / decoded between the
contract and the host
```
fn caller(&self) -> Origin<Self::T>;
-> only use to extract the address of the caller 

fn account_id(&self) -> &AccountIdOf<Self::T>;
 -> only used to expose the address or access the account_id internally   

fn gas_meter(&self) -> &GasMeter<Self::T>;
fn gas_meter_mut(&mut self) -> &mut GasMeter<Self::T>;
 -> encoding does not depend on T

fn call_runtime(&self, call: <Self::T as Config>::RuntimeCall) -> DispatchResultWithPostInfo;
-> Substrate specific, just an opaque blob of bytes from the contract's perspective

fn contract_info(&mut self) -> &mut ContractInfo<Self::T>;
fn transient_storage(&mut self) -> &mut TransientStorage<Self::T>;
-> gated by #[cfg(any(test, feature = "runtime-benchmarks"))]
```

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Co-authored-by: Alexander Theißen <alex.theissen@me.com>