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/postgres.rs

@@ -11,12 +11,50 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Create Kubernetes manager for the database namespace
     let km = KubernetesManager::new("database").await?;
 
+    // Create the namespace if it doesn't exist
+    println!("Creating namespace 'database' if it doesn't exist...");
+    match km.namespace_create("database").await {
+        Ok(_) => println!("✓ Namespace 'database' created"),
+        Err(e) => {
+            if e.to_string().contains("already exists") {
+                println!("✓ Namespace 'database' already exists");
+            } else {
+                return Err(e.into());
+            }
+        }
+    }
+
+    // Clean up any existing resources first
+    println!("Cleaning up any existing PostgreSQL resources...");
+    match km.deployment_delete("postgres-cluster").await {
+        Ok(_) => println!("✓ Deleted existing deployment"),
+        Err(_) => println!("✓ No existing deployment to delete"),
+    }
+
+    match km.service_delete("postgres-cluster").await {
+        Ok(_) => println!("✓ Deleted existing service"),
+        Err(_) => println!("✓ No existing service to delete"),
+    }
+
     // Configure PostgreSQL-specific labels
     let mut labels = HashMap::new();
     labels.insert("app".to_string(), "postgres-cluster".to_string());
     labels.insert("type".to_string(), "database".to_string());
     labels.insert("engine".to_string(), "postgresql".to_string());
 
+    // Configure PostgreSQL environment variables
+    let mut env_vars = HashMap::new();
+    env_vars.insert("POSTGRES_DB".to_string(), "myapp".to_string());
+    env_vars.insert("POSTGRES_USER".to_string(), "postgres".to_string());
+    env_vars.insert(
+        "POSTGRES_PASSWORD".to_string(),
+        "secretpassword".to_string(),
+    );
+    env_vars.insert(
+        "PGDATA".to_string(),
+        "/var/lib/postgresql/data/pgdata".to_string(),
+    );
+
     // Deploy the PostgreSQL cluster using the convenience method
     println!("Deploying PostgreSQL cluster...");
     km.deploy_application(
@@ -25,6 +63,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         2,                  // replicas (1 master + 1 replica)
         5432,               // port
         Some(labels),       // labels
+        Some(env_vars),     // environment variables
     )
     .await?;