feat(ffi): add config server client bindings (#2320)

Add config server client support for the C FFI and Android JNI bindings.

Reuse the existing easytier::web_client::run_web_client path and 
NetworkInstanceManager; OHOS is unchanged.

Report successful remote config apply/delete operations through a 
callback, with one JSON event per affected instance.

Keep the config server client and FFI data plane mutually exclusive: once 
either side is in use, the other side returns an error instead of sharing
lifecycle state.

easytier/src/launcher.rs

@@ -500,6 +500,10 @@ impl NetworkInstance {
         &self.config_file_control
     }
 
+    pub fn get_config(&self) -> TomlConfigLoader {
+        self.config.clone()
+    }
+
     pub fn get_network_config_source(&self) -> ConfigSource {
         self.config.get_network_config_source()
     }