Add c-ffi binding for PSBT signing

[bandrivskiy]

Notes

This PR adds a C-compatible FFI function, rgblib_sign_psbt, which exposes the internal sign_psbt logic
The new binding allowsrgb-lib-nodejs to invoke PSBT signing func via FFI

[zoedberg]

Thanks! Please put the method in alphabetical order and address the small requested change

[zoedberg]

Thanks for addressing the requested changes! Could you please squash the changes in a single commit called c-ffi bindings: add sign_psbt?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.54%. Comparing base (6691268) to head (67c488b).
Report is 2 commits behind head on master.

Additional details and impacted files

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  Coverage   96.54%   96.54%           
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  Files          60       60           
  Lines       20271    20271           
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  Hits        19571    19571           
  Misses        700      700           

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[zoedberg]

LGTM! Thank you!