feat: Add comprehensive API client modules and JWT authentication to Rust SDK

- Add hmac, sha2, and base64 dependencies to Cargo.toml for JWT support
- Add README.md with installation instructions, architecture overview, and API examples for all SDK modules
- Add AuthProvider class with API key and JWT token support (generate_jwt_token, validate_jwt_token, get_headers, clear_token_cache methods)
- Add HTTPClient base class with retry logic, error handling, and timeout management
- Add SubscriberAPI module

sdk/rust/src/api/subscribers.rs

@@ -0,0 +1,67 @@
+use crate::client::HTTPClient;
+use crate::error::TelecomError;
+use crate::types::{
+    CreateSubscriberRequest, Subscriber, SubscriberList, UpdateSubscriberRequest,
+};
+use std::collections::HashMap;
+
+pub struct SubscriberAPI {
+    client: HTTPClient,
+}
+
+impl SubscriberAPI {
+    pub fn new(client: HTTPClient) -> Self {
+        Self { client }
+    }
+
+    pub async fn get(&self, id: i64) -> Result<Subscriber, TelecomError> {
+        self.client.get(&format!("/v1/subscribers/{}", id), None).await
+    }
+
+    pub async fn list(
+        &self,
+        page: i32,
+        page_size: i32,
+        status: Option<String>,
+    ) -> Result<SubscriberList, TelecomError> {
+        let mut params = HashMap::new();
+        params.insert("page".to_string(), page.to_string());
+        params.insert("page_size".to_string(), page_size.to_string());
+        
+        if let Some(status) = status {
+            params.insert("status".to_string(), status);
+        }
+
+        self.client.get("/v1/subscribers", Some(&params)).await
+    }
+
+    pub async fn create(&self, req: &CreateSubscriberRequest) -> Result<Subscriber, TelecomError> {
+        self.client.post("/v1/subscribers", Some(req)).await
+    }
+
+    pub async fn update(
+        &self,
+        id: i64,
+        req: &UpdateSubscriberRequest,
+    ) -> Result<Subscriber, TelecomError> {
+        self.client
+            .put(&format!("/v1/subscribers/{}", id), Some(req))
+            .await
+    }
+
+    pub async fn delete(&self, id: i64) -> Result<(), TelecomError> {
+        self.client.delete(&format!("/v1/subscribers/{}", id)).await
+    }
+
+    pub async fn suspend(&self, id: i64) -> Result<Subscriber, TelecomError> {
+        self.client
+            .post(&format!("/v1/subscribers/{}/suspend", id), None::<()>)
+            .await
+    }
+
+    pub async fn activate(&self, id: i64) -> Result<Subscriber, TelecomError> {
+        self.client
+            .post(&format!("/v1/subscribers/{}/activate", id), None::<()>)
+            .await
+    }
+}