Add `unstable-api` feature flag to `pallet-revive` (#6866)

Follow up refactor to
https://github.com/paritytech/polkadot-sdk/pull/6844#pullrequestreview-2497225717

I still need to finish adding `#[cfg(feature = "unstable-api")]` to the
rest of the tests and make sure all tests pass, I want to make sure I'm
moving into right direction first

@athei @xermicus



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Co-authored-by: DavidK <[email protected]>
Co-authored-by: Alexander Theißen <[email protected]>