RPC endpoint should accept and return block and transaction in JSON format

[shuse2]

Description

Update block and transaction to be returned as JSON format for the fixed part.
Keep header.assets[].data or transaction.params to be encoded so that endpoint remains generic to any modules.

Motivation

Fixed part of the block and transaction can be returned in JSON format.
For example,

{
  "id": "0e4ffc051b353926102eea29ebc4f90c068e81fec380ed7d09f5ebc9ac853ead",
  "senderPublicKey": "7c74abbcf4e52893fc0d785c0d270758210398483bd2ef33343ac4e67d6ae61a",
  "nonce": "23",
  "fee": 100000000",
  "moduleID": 2,
  "commandID": 0,
  params: "3e63d6cf859a64fbd64a"
}

This will allow to read the output at least for the fixed part.

[shuse2]

This is already resolved in #6947