Prevent recipient from introducing mixed input script for only v1, allow in v2

[DanGould]

Enforce this rule on methods that mutate PayjoinProposal. Right now only senders check. I think this basic function is necessary to exit alpha for beta.

edit: would be a lot better to fix the spec in the first place bitcoin/bips#1244

[DanGould]

try_preserving_privacy also accepts any outpoints regardless of their script type. Perhaps that should filter on script as well.