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.

examples/kubernetes/clusters/generic.rs

@@ -11,10 +11,26 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Create Kubernetes manager
     let km = KubernetesManager::new("default").await?;
 
-    // Example 1: Simple web server deployment
-    println!("=== Example 1: Simple Nginx Web Server ===");
+    // Clean up any existing resources first
+    println!("=== Cleaning up existing resources ===");
+    let apps_to_clean = ["web-server", "node-app", "mongodb"];
 
-    km.deploy_application("web-server", "nginx:latest", 2, 80, None)
+    for app in &apps_to_clean {
+        match km.deployment_delete(app).await {
+            Ok(_) => println!("✓ Deleted existing deployment: {}", app),
+            Err(_) => println!("✓ No existing deployment to delete: {}", app),
+        }
+
+        match km.service_delete(app).await {
+            Ok(_) => println!("✓ Deleted existing service: {}", app),
+            Err(_) => println!("✓ No existing service to delete: {}", app),
+        }
+    }
+
+    // Example 1: Simple web server deployment
+    println!("\n=== Example 1: Simple Nginx Web Server ===");
+
+    km.deploy_application("web-server", "nginx:latest", 2, 80, None, None)
         .await?;
     println!("✅ Nginx web server deployed!");
 
@@ -26,12 +42,20 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     node_labels.insert("tier".to_string(), "backend".to_string());
     node_labels.insert("environment".to_string(), "production".to_string());
 
+    // Configure Node.js environment variables
+    let mut node_env_vars = HashMap::new();
+    node_env_vars.insert("NODE_ENV".to_string(), "production".to_string());
+    node_env_vars.insert("PORT".to_string(), "3000".to_string());
+    node_env_vars.insert("LOG_LEVEL".to_string(), "info".to_string());
+    node_env_vars.insert("MAX_CONNECTIONS".to_string(), "1000".to_string());
+
     km.deploy_application(
-        "node-app",        // name
-        "node:18-alpine",  // image
-        3,                 // replicas - scale to 3 instances
-        3000,              // port
-        Some(node_labels), // labels
+        "node-app",          // name
+        "node:18-alpine",    // image
+        3,                   // replicas - scale to 3 instances
+        3000,                // port
+        Some(node_labels),   // labels
+        Some(node_env_vars), // environment variables
     )
     .await?;
 
@@ -45,12 +69,25 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     mongo_labels.insert("type".to_string(), "database".to_string());
     mongo_labels.insert("engine".to_string(), "mongodb".to_string());
 
+    // Configure MongoDB environment variables
+    let mut mongo_env_vars = HashMap::new();
+    mongo_env_vars.insert(
+        "MONGO_INITDB_ROOT_USERNAME".to_string(),
+        "admin".to_string(),
+    );
+    mongo_env_vars.insert(
+        "MONGO_INITDB_ROOT_PASSWORD".to_string(),
+        "mongopassword".to_string(),
+    );
+    mongo_env_vars.insert("MONGO_INITDB_DATABASE".to_string(), "myapp".to_string());
+
     km.deploy_application(
-        "mongodb",          // name
-        "mongo:6.0",        // image
-        1,                  // replicas - single instance for simplicity
-        27017,              // port
-        Some(mongo_labels), // labels
+        "mongodb",            // name
+        "mongo:6.0",          // image
+        1,                    // replicas - single instance for simplicity
+        27017,                // port
+        Some(mongo_labels),   // labels
+        Some(mongo_env_vars), // environment variables
     )
     .await?;