Add TransactionResult to Finality

[nkramer44]

It would be useful to include the validated transaction as a TransactionResult in the Finality class if the transaction that became final was validated on ledger. This should be easy to accomplish because XrplClient.isFinal is already retrieving the TransactionResult. However, currently developers have to call XrplClient.isFinal until the transaction is final, then call XrplClient.transaction to get the validated transaction.