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.go

@@ -0,0 +1,9 @@
+package settings
+
+import "github.com/sagernet/sing-box/adapter"
+
+type WIFIMonitor interface {
+	ReadWIFIState() adapter.WIFIState
+	Start() error
+	Close() error
+}