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-contrib/easytier-android-jni/src/logger.rs

@@ -0,0 +1,13 @@
+use once_cell::sync::Lazy;
+
+static LOGGER_INIT: Lazy<()> = Lazy::new(|| {
+    android_logger::init_once(
+        android_logger::Config::default()
+            .with_max_level(log::LevelFilter::Debug)
+            .with_tag("EasyTier-JNI"),
+    );
+});
+
+pub(crate) fn init() {
+    Lazy::force(&LOGGER_INIT);
+}