feat(coredns): add header plugin

[synackd]

Pull Request Template

Thank you for your contribution! Please ensure the following before submitting:

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
• I have run make test (or equivalent) locally and all tests pass
• DCO Sign-off: All commits are signed off (git commit -s) with my real name and email
• REUSE Compliance:
  • Each new/modified source file has SPDX copyright and license headers
  • Any non-commentable files include a <filename>.license sidecar
  • All referenced licenses are present in the LICENSES/ directory

Description

Please include a summary of the change and which issue is fixed.
Also include relevant motivation and context.

Fixes #(issue)

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

---

For more info, see Contributing Guidelines.

[alexlovelltroy]

This is marked as draft. Do you anticipate additional changes?

[synackd]

I was mulling over whether we wanted to compile in a subset of or the whole set of CoreDNS plugins included in the repo so we don't have to open a PR for every one folks would want. Thoughts?

[alexlovelltroy]

I'm fine with individual PRs as we need more plugins.

[synackd]

What do you think of including import?