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/chain_head.rs

@@ -27,11 +27,11 @@ use crate::{
 		api::ChainHeadApiServer,
 		chain_head_follow::ChainHeadFollower,
 		error::Error as ChainHeadRpcError,
-		event::{FollowEvent, MethodResponse, OperationError, StorageQuery},
-		hex_string,
+		event::{FollowEvent, MethodResponse, OperationError},
 		subscription::{SubscriptionManagement, SubscriptionManagementError},
 	},
-	SubscriptionTaskExecutor,
+	common::events::StorageQuery,
+	hex_string, SubscriptionTaskExecutor,
 };
 use codec::Encode;
 use futures::future::FutureExt;