Make QQuickImageProviderOptions accessible in public API

Many use cases of image providers require access to additional request parameters that are not available when implementing a QQuickImageProvider. Currently, the workaround is to implement the private QQuickImageProviderWithOptions subclass instead. That extends the QQuickImageProvider API with a QQuickImageProviderOptions parameter which contains the additional request data.

This functionality is generally useful and so should be added to the public API in some way. E.g. by making the ...WithOptions class public, or extending QQuickImageProvider with the ...Options parameter, as possible under compat. requirements.