Optionally pass BIP388 policy to signtx #792
                
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Another step towards #785.
Usage:
First register a policy with #791, #647, MooSig or some other method.
Construct a PSBT.
And then try to sign it.
It's easiest to test this with just a single sig wallet. Using #791 you can easily register it and get an hmac (even though registration isn't mandatory).
If you want to try with MuSig, you'll need either #784 or use a Bitcoin Core branch with PSBTv2 support (bitcoin/bitcoin#21283). I still need to test both these approaches.
Strongly overlaps with #647, but this implements signing independent of registration.