feat(gke): add config_sync (fleet) example

[apeabody]

Description

b/388321419

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

Intended location

• Yes, this sample will be (or already is) included on cloud.google.com
  Location(s): https://cloud.google.com/kubernetes-engine/enterprise/config-sync/docs/how-to/installing-config-sync

• 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.

• gke/enterprise/config_sync/git/main.tf:17, tag gke_enterprise_config_sync_git
• gke/enterprise/config_sync/oci/main.tf:17, tag gke_enterprise_config_sync_oci

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