Create transaction: allow someone else to pay the fee

[nicholasmueller]

A new helper like transaction.createTransactionBody... (ie. transaction.createMultiSenderTransaction) that allows someone other than the sender to pay the fee. Can also allow multiple senders in the same function. Guessing its easier/cleaner to create a new function instead of refactor the existing one.

You would have to pass the utxo from the fee payer, and calculate change accordingly. Signing inputs is also affected but i think can be kept separate from this function.