Add health check endpoint to pulp-api

[dkliban]

Add a simple health check endpoint at /pulp/api/v3/healthz/ that returns HTTP 200 with {"status": "ok"} when the API is running.

This endpoint should:

• Not require authentication
• Return a JSON response with {"status": "ok"}
• Be accessible at /pulp/api/v3/healthz/
• Be added as a new view in pulp_service/pulp_service/app/viewsets.py
• Include a URL pattern in pulp_service/pulp_service/app/urls.py