fix: sigstore distributed async signing

[kairoaraujo]

Description

Allows the bundle part for signing

This re-uses the TUFSignatures pydantic structure

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality not to work as expected)

Additional requirements

• [ ] Tests have been added for the bug fix or new feature
• [x] Docs have been added for the bug fix or new feature

Code of Conduct

By submitting this PR, you agree to follow our Code of Conduct.

• [x] I agree to follow this project's Code of Conduct

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.09%. Comparing base (25928b7) to head (44efcaa). Report is 107 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #666 +/- ## ========================================== + Coverage 98.97% 99.09% +0.11% ========================================== Files 14 14 Lines 588 665 +77 ========================================== + Hits 582 659 +77 Misses 6 6 ```

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