Use psetv2 instead of psetv0

[altafan]

Once #125 is closed, we must update ldk version and use the new psetv2 format by default.

[altafan]

There is now a PR in ldk (#140) with ready to use support for psetv2.

@tiero shall we update the existing methods or do we want to be backward compatible and add new apis to make psetv2 swaps?

[tiero]

If possible, I think we should simply support both version of pset: when passed we try to serialize with moth Psbt and Pset classes under the hood