feat(image): Set User-Agent header for Trivy container registry requests

[nabokihms]

Description

• Added functionality to set the User-Agent header in Trivy when making requests to container registries.
• 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 container registries, especially when operating Kubernetes clusters at scale in enterprise environments. It aids in tracking and managing Trivy requests more effectively.

Before the User-Agent header was just a plain go-conainerregistry/version. After the change, it becomes trivy/version go-conainerregistry/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).

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[knqyf263]

Is there any reason for moving the function?

[nabokihms]

To avoid circle dependencies