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/build.rs

@@ -0,0 +1,17 @@
+use std::{env, path::PathBuf};
+
+fn main() {
+    let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
+    if !matches!(target_os.as_str(), "android" | "linux") {
+        return;
+    }
+
+    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
+    let exports = manifest_dir.join("exports.map");
+    println!("cargo:rerun-if-changed={}", exports.display());
+    println!(
+        "cargo:rustc-cdylib-link-arg=-Wl,--version-script={}",
+        exports.display()
+    );
+    println!("cargo:rustc-cdylib-link-arg=-Wl,--exclude-libs,ALL");
+}