RPC-Spec-V2: Backport changes to `chainHead_unstable_storage`

[lexnv]

Implement the following changes to the chainHead_storage RPC method:

• https://github.com/paritytech/json-rpc-interface-spec/pull/37

The chainHead_storage method is now capable of:

• Fetching the value of a key (this is part of the current behavior, both for root trie and child tries)
• Fetching the hash of the value of a key
• Fetching the closest ancestor markle value
• Iterating over keys and values (paginated by the server)
• Iterating over keys and hashes of values (paginated by the server)

// @paritytech/subxt-team

### Tasks
- [ ] https://github.com/paritytech/substrate/issues/14547
- [ ] https://github.com/paritytech/substrate/issues/14548
- [ ] https://github.com/paritytech/substrate/issues/14549
- [ ] https://github.com/paritytech/substrate/issues/14550

### Nice to Have
- [ ] https://github.com/paritytech/substrate/issues/14734

[tomaka]

Fetching the closest ancestor markle value

Hopefully this isn't an issue. I've thought it might be problematic to implement in Substrate, but I don't think we should make the API less powerful just because it's difficult to implement.

You might also want to talk to @cheme, as he was working on https://github.com/paritytech/substrate/pull/12843.

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/new-json-rpc-api-mega-q-a/3048/1

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/new-json-rpc-api-mega-q-a/3048/2