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/graphql.rs

@@ -0,0 +1,23 @@
+use crate::client::HTTPClient;
+use crate::error::TelecomError;
+use crate::types::GraphQLRequest;
+use std::collections::HashMap;
+
+pub struct GraphQLAPI {
+    client: HTTPClient,
+}
+
+impl GraphQLAPI {
+    pub fn new(client: HTTPClient) -> Self {
+        Self { client }
+    }
+
+    pub async fn execute(
+        &self,
+        query: String,
+        variables: Option<HashMap<String, serde_json::Value>>,
+    ) -> Result<HashMap<String, serde_json::Value>, TelecomError> {
+        let req = GraphQLRequest { query, variables };
+        self.client.post("/graphql", Some(&req)).await
+    }
+}