[Feature Request] Add support for signing a PSBT with external inputs

[benalleng]

following #1761 I wanted to open a feature issue to add support for signing a PSBT with external inputs.

It is my understanding that other popular hardware wallets display a warning through this flow but will still sign. This is at the moment the best UX flow that satisfies this condition.

Are there any concerns you might have about the bitbox signing flow regarding this at a high level?

[benma]

I am under the impression that this is not safe without ownership proofs. See the motivation section here:

https://github.com/satoshilabs/slips/blob/slips-19-20-coinjoin-proofs/slip-0019.md#motivation

Does payjoin provide ownership proofs?

[benalleng]

While payjoin does not explicitly use a proof to handle this we have a check_inputs_not_owned state that validates there are none of the receiver's inputs trying to be spent.
https://github.com/payjoin/rust-payjoin/blob/67b14ded225f99758b13f222d4531621a536194d/payjoin/src/core/receive/mod.rs#L388-L413