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 --------- Signed-off-by: Alexandru Vasile <[email protected]> Co-authored-by: James Wilson <[email protected]>