Add existing private dns linking for vnet

[ChrisChapman-gh]

Description

Closes #60
Depends on #50

Adds linking between the created vnet and existing private dns zones.
Moves the platform + created/existing zone resource Id logic to the locals.
Removed the need for a dummy value should created/existing not be required. Either the private endpoints are managing the
dns zone group, in which case an existing or created id should be present, or they are unmanaged and a null/[] is passed and ignored.

Type of Change

• Non-module change (e.g. CI/CD, documentation, etc.)
• Azure Verified Module updates:
  • Bugfix containing backwards compatible bug fixes
    • 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.
  • Feature update backwards compatible feature updates.
  • Breaking changes.
  • Update to documentation

Checklist

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