Refactor WLAN PSK retrieval code

- Reduce platform specific code in ImageWriter class,
  and move that to seperate classes.
- Use API calls to get current SSID on Windows and Linux instead
  of launching command line utilities.

src/wlancredentials.h

@@ -0,0 +1,26 @@
+#ifndef WLANCREDENTIALS_H
+#define WLANCREDENTIALS_H
+
+/*
+ * Interface for wlan credential detection
+ * Use WlanCredentials::instance() to get platform
+ * specific implementation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2023 Raspberry Pi Ltd
+ */
+
+#include <QByteArray>
+
+class WlanCredentials
+{
+public:
+    static WlanCredentials *instance();
+    virtual QByteArray getSSID() = 0;
+    virtual QByteArray getPSK() = 0;
+
+protected:
+    static WlanCredentials *_instance;
+};
+
+#endif // WLANCREDENTIALS_H