Add deprecated warnings

experimental/deprecated/manager.go

@@ -0,0 +1,19 @@
+package deprecated
+
+import (
+	"context"
+
+	"github.com/sagernet/sing/service"
+)
+
+type Manager interface {
+	ReportDeprecated(feature Note)
+}
+
+func Report(ctx context.Context, feature Note) {
+	manager := service.FromContext[Manager](ctx)
+	if manager == nil {
+		return
+	}
+	manager.ReportDeprecated(feature)
+}