feat(sbom): set User-Agent header on requests to Rekor

[bobcallaway]

Description

• Added functionality to set the User-Agent header in Trivy when making requests to Sigstore's Rekor transparency log.
• Updated tests to cover the new functionality and ensure that the User-Agent header is properly set.

This enhancement is important for distinguishing requests to Rekor. It aids in tracking and managing Trivy requests more effectively.

Before the User-Agent header was just a plain Go-http-client/2.0. After the change, it becomes trivy/version.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

Thanks for your contribution. I triggered tests.