feat: Enable RSTUF API to use SigStore for Signatures

[kairoaraujo]

Description

This commit enables RSTUF API to use SigStore signatures in the TUF Metadata.

When using SigStore, the Signature can contain the bundle.

The KeyVal format can contain the issuer and identity used by SigStore

This PR enables API for https://github.com/repository-service-tuf/repository-service-tuf-cli/issues/657

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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 99.10%. Comparing base (25928b7) to head (e058ac8). Report is 106 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #664 +/- ## ========================================== + Coverage 98.97% 99.10% +0.12% ========================================== Files 14 14 Lines 588 668 +80 ========================================== + Hits 582 662 +80 Misses 6 6 ```

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