rpc-v2: Implement `archive_unstable_storageDiff` (#5997)

This PR implements the `archive_unstable_storageDiff`.

The implementation follows the rpc-v2 spec from:
-  https://github.com/paritytech/json-rpc-interface-spec/pull/159.
- builds on top of
https://github.com/paritytech/json-rpc-interface-spec/pull/161

cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: James Wilson <james@jsdw.me>