RPC-Spec-V2: Backport changes to chainHead_unstable_storage

[lexnv]

Implement the following changes to the chainHead_storage RPC method:

• Make chainHead_unstable_storage more powerful json-rpc-interface-spec#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

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1. [RPC-Spec-V2] Storage: Backport queries for value and hash of value #14547
   J0-enhancement +1
   
2. [RPC-Spec-V2] Storage: Add iteration support for storage queries #14548
   J0-enhancement +1
   
3. [RPC-Spec-V2] Storage: Add support for pagination #14549
   J0-enhancement +1
   
4. [RPC-Spec-V2] Storage: Add support for storage closest merkle descendant #14550
   J0-enhancement +1
   

Nice to Have

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1. [RPC-Spec-V2] Limit chainHead_unstable_follow subscriptions #14734
   J0-enhancement +1
   

[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 #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