Add a SIGHASH_DOUBE flag

The conversation suggests that if modifications are required for implementing soft-fork changes to SIGHASH, it might be advantageous to also include index information beneficial for PSBTs. This approach prompts further inquiry into what steps would be necessary to progress this idea into a formal proposal suitable for inclusion in a soft fork alteration. The dialogue indicates a clear interest in exploring the technical and procedural requirements needed to advance this concept within the context of Bitcoin's development framework.