Implement a wrapper for getblock with verbosity value 2

[danielost]

Changes

• Add get_block_txs method that calls getblock with verbosity value 2
• Add GetBlockTxResult struct to json
• Utilize composition in GetBlockResult and GetBlockTxResult to get rid of duplicate fields

[0xB10C]

I was excited to see this when looking for a way to call getblock <hash> 2 to get the transaction fee of transactions it that block. However, the proposed implementation only deserializes the the hex field of each transaction. This is essentially the same as calling get_block_info() (if information about e.g. the next block or the number of confirmations are wanted) and get_block() for the raw transactions without any metadata.