Simplify the types that `ink_e2e` requires for balances

[cmichi]

We currently "forward" the two types that pallet-revive uses for balances.

It's either U256 or the configured pallet_revive::Config::Currency. The latter is basically the Polkadot SDK currency of the runtime and the former the Eth-compatible one. There is a calculation factor pallet_revive::Config::NativeToEthRatio that is used to convert between the two.

This poses some unnecessary user friction, as e.g. in our E2E tests some methods require U256 while others need ink::Environment::Balance. We should instead decide on one type and do the conversion via NativeToEthRatio in the backend.

[evilrobot-01]

@chungquantin is working on this

[cmichi]

Duplicate of https://github.com/use-ink/ink/issues/2597, closing.