Add Linux WI-FI state support

Support monitoring WIFI state on Linux through:
- NetworkManager (D-Bus)
- IWD (D-Bus)
- wpa_supplicant (control socket)
- ConnMan (D-Bus)

common/settings/wifi_stub.go

@@ -0,0 +1,27 @@
+//go:build !linux
+
+package settings
+
+import (
+	"os"
+
+	"github.com/sagernet/sing-box/adapter"
+)
+
+type stubWIFIMonitor struct{}
+
+func NewWIFIMonitor(callback func(adapter.WIFIState)) (WIFIMonitor, error) {
+	return nil, os.ErrInvalid
+}
+
+func (m *stubWIFIMonitor) ReadWIFIState() adapter.WIFIState {
+	return adapter.WIFIState{}
+}
+
+func (m *stubWIFIMonitor) Start() error {
+	return nil
+}
+
+func (m *stubWIFIMonitor) Close() error {
+	return nil
+}