feat: add Media CDN sample for custom headers

[kevpong]

Description

Fixes #b/402141524

Note: If you are not associated with Google, open an issue for discussion before submitting a pull request.

Checklist

Readiness

• Yes, merge this PR after it is approved
• No, don't merge this PR after it is approved

Style

• My sample follows the rules described for Terraform in the Effective Samples style guide
• My sample follows the other requirements and best practices in the Contributing
  guide

Testing

• I have performed tests described in the Contributing guide:

  • Tests pass: terraform apply
  • Lint pass: terraform fmt check

Note: I tested in my own project per https://cloud.google.com/guides/product-specific/terraform/git-terraform#own-project.

Intended location

• Yes, this sample will be (or already is) included on cloud.google.com
  Location(s): https://cloud.google.com/media-cdn/docs/custom-headers

• No, this sample won't be included on cloud.google.com
  Reason:

API enablement

• If the sample needs an API enabled to pass testing, I have added the service to the Test setup file

Review

• If this sample adds a new directory, I have added codeowners to the CODEOWNERS file

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• media_cdn/custom_headers/main.tf:22, tag mediacdn_custom_headers_parent_tag
• media_cdn/custom_headers/main.tf:68, tag mediacdn_custom_headers_route

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[gericdong]

/gcbrun