Implement BIP-375 Silent Payments PSBT Fields

[notTanveer]

Key Changes:

• New PSBT Fields:
  • Global:
    • PSBT_GLOBAL_SP_ECDH_SHARE (0x07)
    • PSBT_GLOBAL_SP_DLEQ (0x08)
  • Per-Input:
    • PSBT_IN_SP_ECDH_SHARE (0x1d)
    • PSBT_IN_SP_DLEQ (0x1e)
  • Per-Output:
    • PSBT_OUT_SP_V0_INFO (0x09)
    • PSBT_OUT_SP_V0_LABEL (0x0a)
• Parsing and Serialization:
  • Implemented parsing logic in PSBT.parse_unknowns, InputScope.read_value, and OutputScope.read_value for the new fields, including key and value length validations.
  • Implemented serialization logic in PSBT.write_to, InputScope.write_to, and OutputScope.write_to.
  • Silent Payment specific fields (global, per-input, per-output) are serialized only for PSBT version 2, as per BIP-375 requirements.
• PSBT Version Handling:
  • The PSBT class now explicitly handles version (None for v0, 2 for v2).
  • Global Silent Payment fields will raise an error if encountered or serialized in a non-v2 PSBT.
• Signer Logic (PSBT.sign_with):
  • Added checks to prevent signing if inputs with Segwit version > 1 are present alongside Silent Payment outputs.
  • Enforces SIGHASH_ALL for inputs when Silent Payment outputs are present.
• Output Handling:
  • The OutputScope.vout property now correctly handles cases where script_pubkey might be None if sp_v0_info is present (indicating a Silent Payment output whose script is not yet computed).
• Backwards Compatibility:
  • Ensured that existing PSBTv0 and Taproot field serialization remains correct, particularly for Taproot output fields in OutputScope.write_to after version-conditional logic was added.

still figuring out the tests for this implementation(BIP-375 has no test vectors) — will add them soon.

[notTanveer]

BIP 375 is still in draft and doesn’t have a reference implementation yet, mainly because folks are still figuring out the best way to handle it. so i’m converting this to a draft PR for now. once the test vectors and other pieces are in place, we can revisit and move it forward.