Make the asset downloading functionality reusable across examples

The Car Configurator demo implements an AssetDownloader class that allows the example to fetch large 3D assets from online storage. This helps with keeping the size of our packages small, and also allows us to update such assets independent of Qt releases.

Making this functionality generally available to examples would be very useful, as both 2D and 3D assets are becoming increasingly important for many of our examples.

We should move the AssetDownloader into a library (could be a static library) that can be reused by different examples, with a small API to allow for basic configuration.

Ideally, the AssetDownloader can be available to examples that don't depend on anything else than qtbase; it's currently registered as a QML type, but otherwise has no dependencies outside of QtCore and QtNetwork, so having the QML type as a very thin wrapper in a QML module should be possible.