[RPC-Spec-V2] Backport changes to chainHead

[lexnv]

Implement the following changes to the chainHead class of RPC methods:

• Change the chainHead functions to have proper limits json-rpc-interface-spec#66

The chainHead_follow is now responsible for generating the result of the other methods:

• chainHead_follow events / errors are extended to announce blocks and results of other methods
• chainHead_storage, chainHead_call, chainHead_body are plain methods that return an operation ID
• chainHead_follow produces the response of the provided operations
• ongoing operations can be canceled by a new method chainHead_stopOperation
• chainHead introduces a minimum 16 number of ongoing operations after which the limitReached event is produced

// @paritytech/subxt-team

Tasks

Beta Give feedback

1. RPC-Spec-V2: Backport changes to chainHead_unstable_storage #14222
   4 of 5
   J0-enhancement +1
   
2. [RPC-Spec-V2] Sync chainHead events and reported errors with spec #14637
   J0-enhancement +1
   
3. [RPC-Spec-V2] Extend chainHead_follow with method responses #14638
   J0-enhancement +1
   
4. [RPC-Spec-V2] Cancel ongoing operations #14639
   J0-enhancement +1
   
5. [RPC-Spec-V2] Limit ongoing operations #14640
   J0-enhancement +1