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/tests.rs

@@ -16,9 +16,10 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
-use crate::chain_head::{
-	event::{MethodResponse, StorageQuery, StorageQueryType, StorageResultType},
-	test_utils::ChainHeadMockClient,
+use crate::{
+	chain_head::{event::MethodResponse, test_utils::ChainHeadMockClient},
+	common::events::{StorageQuery, StorageQueryType, StorageResultType},
+	hex_string,
 };
 
 use super::*;