QMediaPlayer: Provide API to set network stream buffer size

https://doc.qt.io/qt-6/qmediaplayer.html#bufferProgress-prop says, "The buffer will need to reach 100% filled before playback can start or resume". When streaming media over a network, it would be useful to have the ability to:

• Increase the buffer size, to facilitate smooth playback on poorer connections.
• Decrease the buffer size, to reduce the delay before playback starts.

For reference, VLC Media Player allows users to set "caching" time: https://stackoverflow.com/questions/19264081/caching-time-and-frame-rate-in-video-streaming-using-vlc-media-player