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/lib.rs

@@ -24,6 +24,9 @@
 #![deny(unused_crate_dependencies)]
 
 use serde::{Deserialize, Serialize};
+use sp_core::hexdisplay::{AsBytesRef, HexDisplay};
+
+mod common;
 
 pub mod archive;
 pub mod chain_head;
@@ -39,7 +42,7 @@ pub type SubscriptionTaskExecutor = std::sync::Arc<dyn sp_core::traits::SpawnNam
 pub enum MethodResult {
 	/// Method generated a result.
 	Ok(MethodResultOk),
-	/// Method ecountered an error.
+	/// Method encountered an error.
 	Err(MethodResultErr),
 }
 
@@ -75,6 +78,11 @@ pub struct MethodResultErr {
 	pub error: String,
 }
 
+/// Util function to encode a value as a hex string
+pub fn hex_string<Data: AsBytesRef>(data: &Data) -> String {
+	format!("0x{:?}", HexDisplay::from(data))
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;