PSBT Version 2.

Updates the PSBT file format for unsigned payments so inputs and outputs can still be added after the file is first made.

A PSBT is a file that carries an unfinished payment between wallets and signing devices until everyone has signed. The first version fixed the payment's inputs and outputs at the start. Version 2 lets more be added later, which helps when several people build one payment, and keeps each input's details in one place.