[router] Add IGW (Inference Gateway) Feature Flag (#9371)

Co-authored-by: Yineng Zhang <me@zhyncs.com>

sgl-router/tests/request_formats_test.rs

@@ -42,6 +42,7 @@ impl TestContext {
             disable_retries: false,
             disable_circuit_breaker: false,
             health_check: sglang_router_rs::config::HealthCheckConfig::default(),
+            enable_igw: false,
         };
 
         let mut workers = Vec::new();