Add full support for BIP-370 (PSBTv2)

[notTanveer]

Currently, embit provides only partial support for PSBT v2 as defined in BIP-370, with most input fields unimplemented and lacking test coverage.

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Observations

1. No Explicit GlobalScope Class

• embit currently does not define a dedicated GlobalScope class.
• Global fields are handled directly in the PSBT class through attributes like:
  • self.unknown
  • self.xpubs
  • self.tx_version
  • self.locktime
  • and utility methods such as parse_unknowns().
• This diverges from BIP-370, which specifies an explicit <globals> scope, distinct from inputs and outputs.

2. Partial PSBTv2 Field Support

• InputScope and OutputScope have limited support for PSBTv2 fields (e.g., txid, vout, sequence, value, script_pubkey), conditionally handled when version == 2.
• However, many new or optional v2 fields from BIP-370 are not implemented.
• The current structure is more v0-centric, with v2 fields sprinkled in, rather than fully modular as per the spec.

3. Implementation Summary

Scope	Dedicated Class	BIP-370 v2 Fields Supported?	Notes
GlobalScope	❌	Partially	Managed inside the PSBT class
InputScope	✅	Partially	Some v2 fields supported; others missing
OutputScope	✅	Partially	Similar situation to InputScope

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Proposed Plan

• Refactor to introduce explicit GlobalScope support (optional but encouraged for modularity)
• Implement full parsing and serialization for all defined BIP-370 fields
• Add support for remaining v2-only fields across Input and Output scopes
• Validate against BIP-370 test vectors
• Add comprehensive unit tests to verify behavior

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Happy to discuss the design direction and contribute the necessary changes! Let me know if there's already ongoing work I should align with.

[jdlcdl]

Just a note that other bip370 psbt-v2 fields are also not implemented in embit's InputScope and OutputScope (and that there is no implementation like: GlobalScope in embit).

[notTanveer]

The design is more v0-centric with v2 field support sprinkled in, rather than a full BIP370-compliant structure.
btw im currently working on a PR for this. (updated the issue)