Qt WebView: Provide a way to enable persistent cookies on desktop platforms

In Qt 6, the default QWebEngineProfile is now off-the-record (https://doc.qt.io/qt-6/qtwebengine-changes-qt6.html#default-profile ). Consequently, Qt WebView on desktop platforms can no longer store cookies on disk.

• There is currently no Qt WebView API to enable persistent cookies on desktop platforms (or to disable them on mobile platforms).
• While it is possible to obtain the default profile by calling `QWebEngineProfile::defaultProfile()`, it is impossible to disable off-the-record mode on an existing profile.

Suggestion

Provide a "Native Interface" API to enable applying a custom profile to the underlying WebEngineView (see https://doc.qt.io/qt-6/native-interfaces.html, https://doc.qt.io/qt-6/qguiapplication.html#nativeInterface )