Improve Coin Selection API

[DanGould]

Right now our only coin selection is try_preserving_privacy, which is supposed to attempt to avoid UIH and if not use the first output. In practice, the select_first_candidate only runs on sweeps, since avoid_uihthrows an error if there are 2 outputs in the original psbt.

In practice, sometimes you want to select the first candidate no matter once since you have a discounted opportunity to conolidate a receiver input.

@SatoshiPortal suggested a possible "consolidation" selection algorithm that tried to optimize for consolidation that could be enabled when fees were below a certain threshold. We can support something like this.

I think this is something @0xBEEFCAF3 is interested in