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/web_client/mod.rs

@@ -23,6 +23,10 @@ use uuid::Uuid;
 
 #[async_trait]
 pub trait WebClientHooks: Send + Sync {
+    fn manages_remote_config_instances(&self) -> bool {
+        false
+    }
+
     async fn pre_run_network_instance(&self, _cfg: &TomlConfigLoader) -> Result<(), String> {
         Ok(())
     }