Implement writing of the output of QMediaRecorder to QIODevice

The API QMediaRecorder::setOutputDevice has been added, the task is to implement it for the ffmpeg backend.

Possible steps:
1. Move ffmpeg IO helpers from to qffmpegavioutils_p.h to the new file qffmpegmediadataholder
2. Implement writing writeQIODevice in the added files.
3. Propage QIODevice* from the interface to the encoding engine through virtual QPlatformMediaRecorder::setOutputDevice.
4. In the constructor of encoder (or RecordingEngine after refactorings), if IODevice is set, initialize context->pb with avio_alloc_context instead of opening url.