feat(GKE): add binary-authorization enforce sample

[apeabody]

Description

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

Checklist

Readiness

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

Style

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

Testing

• [x] I have performed tests described in the Contributing guide:

  • [x] Tests pass: terraform apply
  • [x] Lint pass: terraform fmt check

Intended location

• [x] Yes, this sample will be (or already is) included on cloud.google.com Location(s): https://cloud.google.com/binary-authorization/docs/creating-cluster

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

API enablement

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

Review

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

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [gke/standard/regional/binary-authorization/main.tf:17](https://github.com/terraform-google-modules/terraform-docs-samples/blob/6e9796e516a9a7763b84fc6b4d6dba53c87e21c6/gke/standard/regional/binary-authorization/main.tf#L17), tag `gke_standard_regional_binauthz_enforce`

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