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-ffi/src/strings.rs

@@ -0,0 +1,23 @@
+use std::ffi::{CStr, c_char};
+
+pub(crate) unsafe fn c_str_to_string(ptr: *const c_char, name: &str) -> Result<String, String> {
+    if ptr.is_null() {
+        return Err(format!("{} is null", name));
+    }
+
+    unsafe { CStr::from_ptr(ptr) }
+        .to_str()
+        .map(|value| value.to_string())
+        .map_err(|err| format!("{} is not valid UTF-8: {}", name, err))
+}
+
+pub(crate) unsafe fn optional_c_str_to_string(
+    ptr: *const c_char,
+    name: &str,
+) -> Result<Option<String>, String> {
+    if ptr.is_null() {
+        return Ok(None);
+    }
+
+    unsafe { c_str_to_string(ptr, name) }.map(Some)
+}