Have `try_preserving_privacy` modify the PSBT

[DanGould]

Right now it just does coin selection which may be applied using contribute methods. It would be more convenient to apply the selection in one call.

[spacebear21]

The coin selection and contribution steps were kept distinct, but #377 modified the try_preserving_privacy signature to be much more ergonomic and the resulting type can be applied directly to contribute_inputs.