Add cluster extension support with documentation and examples

[matt-FFFFFF]

This PR replaces #157 to enable CI/CD checks with required secrets.

Original PR by @Pyodin

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Description

Introduce support for cluster extensions in Terraform configurations, including detailed documentation and an example for AKS. This enhancement allows users to define and manage Kubernetes cluster extensions effectively.

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

[microsoft-github-policy-service]

Important

The "Needs: Triage 🔍" label must be removed once the triage process is complete!

Tip

For additional guidance on how to triage this issue/PR, see the Terraform Issue Triage documentation.