Merge #546: psbt: implement BIP 371 Taproot PSBT fields

c5fba03 tests: Add Taproot field test vectors (Andrew Chow)
19d2e0a psbt: Implement Taproot fields (Andrew Chow)
fe85a31 psbt: Use static member field numbers for serialization (Andrew Chow)
6b59c75 psbt: Use static members for field numbers (Andrew Chow)
bc7efb9 psbt: Make sighash Optional (Andrew Chow)
92d64e7 psbt: Remove extra newlines (Andrew Chow)
76ad4dd psbt: Check for duplicate keys by tracking set of read keys (Andrew Chow)

Pull request description:

  Adds the Taproot PSBT fields as described in BIP 371

ACKs for top commit:
  Sjors:
    re-utACK c5fba03

Tree-SHA512: 93505dbad541324c9f419888134fe9543bccba603a863eed8561a0b514f0b77a6e27160222f90b8011e9e46f34d9e3897dce78739268a339204873939a48a6db