feat: use sweep_target_address in spendable tx

[EthanYuan]

Passed to the mutiny lib using sweep_target_address, the mutiny node's background thread is automatically transferred to the target address when the channel is closed and some of the outputs satisfy the condition triggering the Event::SpendableOutputs event.

This eliminates the need to manually initiate a sweep tx and saves transaction fees.

For the caller, sweep_target_address in the MutinyWallet:: new parameter is similar to destination_address in MutinyWallet:: sweep_wallet, except for the additional Option wrapping. If None is passed, the behavior is the same as before.