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/error.rs

@@ -0,0 +1,22 @@
+use reqwest::StatusCode;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum TelecomError {
+    #[error("Authentication failed")]
+    AuthenticationError,
+    #[error("API error: {0}")]
+    APIError(String),
+    #[error("Network error: {0}")]
+    NetworkError(#[from] reqwest::Error),
+    #[error("Validation error: {0}")]
+    ValidationError(String),
+    #[error("Rate limit exceeded")]
+    RateLimitError,
+    #[error("Server error: {0}")]
+    ServerError(StatusCode),
+    #[error("WebSocket error: {0}")]
+    WebSocketError(String),
+    #[error("JSON error: {0}")]
+    JsonError(#[from] serde_json::Error),
+}