feat(ci): add CMX cluster validation workflow

[kriscoleman]

Summary

This PR introduces a new GitHub Actions workflow that tests kURL deployments on Compatibility Matrix (CMX) clusters. It creates a real kURL cluster via the replicated CLI, validates provisioning, and ensures the cluster is functional before cleanup.

Changes

• Added .github/workflows/test-cmx-clusters.yaml
• Workflow runs on workflow_dispatch and daily at 06:00 UTC via cron schedule
• Installs replicated CLI and creates a kURL CMX cluster (r1.large instance type)
• Validates cluster health by:
  • Retrieving kubeconfig and listing nodes, namespaces, pods, and services
  • Verifying the kurl namespace exists
  • Waiting for all nodes to be Ready
  • Testing DNS resolution within the cluster
• Implements automatic cleanup with always() condition to remove CMX clusters even if tests fail
• Restricted to run only on the replicatedhq organization

UAT

1. Go to the Actions tab in the repository
2. Select test-cmx-clusters workflow
3. Click Run workflow and trigger it manually
4. Verify that:
   • The CMX cluster is created successfully
   • Validation steps pass (nodes ready, DNS works, kurl namespace exists)
   • Cluster is removed automatically after completion (check the cleanup step)

Fixes: [sc-137574]

[xavpaice]

Update: I've removed the schedule from the workflow, there's a followup PR to make that triggers this workflow on release. What I'd like to do is have this reviewed and merged, then test the workflow (can't test unless the file is in Main), prior to automating the trigger.