feat: add spend_utxo to Psbt

[ItoroD]

Description

Exposing spend_utxo will be helpful when trying to validate signatures or sign transaction inputs. This will allows signers of a psbt to verify and confirm the amount being spent and ensure it is what they expect before they approve/sign the transaction.

Notes to the reviewers

spend_utxo returns a SignError as part of result in case of any errors. We do not have this error type so I skipped this. After testing we still get a fairly descriptive error message when an error occurs. At least for passing an out of bounds index.

Changelog notice

• Check Psbt utxo at given index

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[reez]

ACK f1f25d1

[reez]

just waiting for ci to finish (python tests have been stuck queued for a little while so far) before I merge