Add unmanaged_dns_zone_groups block with switch

[chinthakaru]

Description

Update unmanaged_dns_zone_groups resource block and and let Azure Policy to manage existing Azure private DNS zone.

Fixes #138 Closes #138

Type of Change

• [ ] Non-module change (e.g. CI/CD, documentation, etc.)
• [ x] Azure Verified Module updates:
  • [ x] Bugfix containing backwards compatible bug fixes
  • [ x] Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
  • [ ] The bug was found by the module author, and no one has opened an issue to report it yet.
  • [x ] Feature update backwards compatible feature updates.
  • [ ] Breaking changes.
  • [ x] Update to documentation

Checklist

• [ x] I'm sure there are no other open Pull Requests for the same update/change
• [ x] My corresponding pipelines / checks run clean and green without any errors or warnings
• [ x] I did run all pre-commit checks

[thenewnano]

How it the coordination between you and other AVM module maintainers, because if this files it should be ported to all other modules? as we are using a few of those, and totally gave up private endpoint management by them and deploy our own module for that, because of policy managed DNS.