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_linux.go

@@ -0,0 +1,46 @@
+package settings
+
+import (
+	"github.com/sagernet/sing-box/adapter"
+	E "github.com/sagernet/sing/common/exceptions"
+)
+
+type LinuxWIFIMonitor struct {
+	monitor WIFIMonitor
+}
+
+func NewWIFIMonitor(callback func(adapter.WIFIState)) (WIFIMonitor, error) {
+	monitors := []func(func(adapter.WIFIState)) (WIFIMonitor, error){
+		newNetworkManagerMonitor,
+		newIWDMonitor,
+		newWpaSupplicantMonitor,
+		newConnManMonitor,
+	}
+	var errors []error
+	for _, factory := range monitors {
+		monitor, err := factory(callback)
+		if err == nil {
+			return &LinuxWIFIMonitor{monitor: monitor}, nil
+		}
+		errors = append(errors, err)
+	}
+	return nil, E.Cause(E.Errors(errors...), "no supported WIFI manager found")
+}
+
+func (m *LinuxWIFIMonitor) ReadWIFIState() adapter.WIFIState {
+	return m.monitor.ReadWIFIState()
+}
+
+func (m *LinuxWIFIMonitor) Start() error {
+	if m.monitor != nil {
+		return m.monitor.Start()
+	}
+	return nil
+}
+
+func (m *LinuxWIFIMonitor) Close() error {
+	if m.monitor != nil {
+		return m.monitor.Close()
+	}
+	return nil
+}