feat: Allow override of containerd namespace

[pmengelbert]

Description

Containerd allows you to specify a namespace for queries, pulls, storage, etc. Trivy currently only scans local images in the default containerd namespace. This PR enables trivy to scan other namespaces within the containerd store.

An excellent use case is scanning the containerd store on kubernetes clusters; by default, images used by kubernetes are pulled to the k8s.io namespace.

Related issues

• Close Support CONTAINERD_NAMESPACE environment variable #3059

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) does not introduce new options
• I've included a "before" and "after" example to the description (if the PR is a user interface change). not a UI change

[CLAassistant]

All committers have signed the CLA.

[pmengelbert]

Had to rebase after main branch updated. Will need re-approval for the integration test workflow, please and thanks :)

[knqyf263]

Thanks for your contribution! Do you mind if I ask you to resolve the conflict?

[pmengelbert]

Thanks for your contribution! Do you mind if I ask you to resolve the conflict?

@knqyf263 Done! Will you please re-approve the test workflow? Thanks

[knqyf263]

Thanks!