archive: Implement archive_unstable_storage (#1846)

This PR implements the `archive_unstable_storage` method that offers
support for:
- fetching values
- fetching hashes
- iterating over keys and values
- iterating over keys and hashes
- fetching merkle values from the trie-db

A common component dedicated to RPC-V2 storage queries is created to
bridge the gap between `chainHead/storage` and `archive/storage`.
Query pagination is supported by `paginationStartKey`, similar to the
old APIs.
Similarly to the `chainHead/storage`, the `archive/storage` method
accepts a maximum number of queried items.

The design builds upon:
https://github.com/paritytech/json-rpc-interface-spec/pull/94.
Closes https://github.com/paritytech/polkadot-sdk/issues/1512.

cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

substrate/client/rpc-spec-v2/src/chain_head/mod.rs

@@ -42,10 +42,3 @@ pub use event::{
 	BestBlockChanged, ErrorEvent, Finalized, FollowEvent, Initialized, NewBlock, RuntimeEvent,
 	RuntimeVersionEvent,
 };
-
-use sp_core::hexdisplay::{AsBytesRef, HexDisplay};
-
-/// Util function to print the results of `chianHead` as hex string
-pub(crate) fn hex_string<Data: AsBytesRef>(data: &Data) -> String {
-	format!("0x{:?}", HexDisplay::from(data))
-}