Add support for sighash flags in PSBT (like SINGLE|ANYONECANPAY)

[orenyomtov]

Is your feature request related to a problem? Please describe.
When using Bitcoin Core's GUI to sign a PSBT that has a UTXO with sighash=SINGLE|ANYONECANPAY, it throws following error:

Specified sighash value does not match value stored in PSBT

Describe the solution you'd like
I'd like it to successfully sign PSBTs containing sighash=SINGLE|ANYONECANPAY

Describe alternatives you've considered
The current non-idea workaround is to use the console window as described here:
orenyomtov/openordex#1 (comment)

Additional context
Example PSBT:

cHNidP8BAF4CAAAAAXeYThreN2BAK3ExrpqGUIX/ZZZnz3ICQcL0HdIyNBraAAAAAAD/////ARCkAAAAAAAAIlEgVWMpPJTgiS8bEHeTZiizvHOP7LskZAgXa50KvKsv/2IAAAAAAAEAyAIAAAABtliktgpzhuZ4feeNzIMfd1gmwpdZVivt3Fs9jE62veQAAAAAakcwRAIgSRen/81qQ0C5MVYtZQjRa8zkXzv4TIAnCxBQgUHeqUECICINPEFntasthDMzMOryuQFWuHvJ8GUA3o0blSKi8kRXASEDs2NwiId+uHhKaiiSgNi+bs+C0mPlQzoLte4Vz8l4lOP9////AWweAAAAAAAAIlEgVWMpPJTgiS8bEHeTZiizvHOP7LskZAgXa50KvKsv/2LL3QsAAQMEgwAAAAAA

(taken from https://openordex.org/inscription?number=151969)

Originally opened here bitcoin/bitcoin#27141
But I've been told this repo is the better fit