[Feature]: Define the appropriate json structure of BlockHeader and Tx in the listener operations of server/v2

[cool-develope]

Context

There are some todos in server/v2 and baseapp listener staff

• need to determine the serialization structure of BlockHeader in listener.StartBlock
• need to determine the json structure of tx in listener.OnTx

[facundomedica]

For now we'll just use the default json marshalling