feat: Add Kubernetes examples and update dependencies

- Add Kubernetes examples demonstrating deployment of various
  applications (PostgreSQL, Redis, generic). This improves the
  documentation and provides practical usage examples.
- Add `tokio` dependency for async examples. This enables the use
  of asynchronous operations in the examples.
- Add `once_cell` dependency for improved resource management in
  Kubernetes module. This allows efficient management of
  singletons and other resources.

rhai_tests/kubernetes/05_production_safety.rhai

@@ -1,5 +1,3 @@
-#!/usr/bin/env rhai
-
 // Test 5: Production Safety Features
 // This test covers timeouts, rate limiting, retry logic, and safety features