add enum for PIN state

src/enums/NukiPinState.h

@@ -0,0 +1,9 @@
+#pragma once
+
+enum class NukiPinState
+{
+    NotSet = 0,
+    Valid = 1,
+    Invalid = 2,
+    NotConfigured = 4 // default value used for preferences.getInt() when not configured by user
+};