sender: allow for multiple payouts

[0xBEEFCAF3]

original TODO comment:

rust-payjoin/payjoin/src/send/mod.rs

Line 105 in 6e512ff

// TODO support optional batched payout scripts. This would require a change to

Allow a sender to be created that pays out to multiple destination addresses. We should replace check_single_payee with check_for_all_payouts and unit test it

[DanGould]

This kind of multiparty API change needs to be experimented with before payjoin-1.0 so we know that our stable API can accommodate it. It would be crappy to stabilize the API and realize multiparty required a complete re-implementation with a different API.