BIP-174: add SLIP-0019 fields

[louneskmt]

The PSBT extension section of the SLIP-0019 has just been clarified and updated in satoshilabs/slips@019bcd6.

[murchandamus]

@achow101: could you review this when you have a chance?

[achow101]

Tend to NACK. IMO every non-proprietary PSBT field should be defined in a BIP. If you don't want to do that, use a proprietary field. I prefer to be very strict with that as otherwise we open up people to start defining their own fields and someone having to go and hunt down every unBIP'd usage of a field number, which is just an insane maintenance burden. We also want to avoid conflicting field numbers, so requiring everything to be a BIP will actually standardize what each field number means.

Note that field number 0x19 collides with proposed field numbers in #1540

[jonatack]

Closing per BIP author feedback (thanks @achow101).