Add support for AV1 decoding with the FFmpeg backend in online installer

The FFmpeg multimedia backend that comes with the online installer does not support decoding AV1 video files. It is, however, possible to configure FFmpeg to enable the libdav1d decoder (https://code.videolan.org/videolan/dav1d) and enable AV1 playback with a custom build of the FFmpeg media backend.

The need for AV1 playback is documented in QTBUG-117118. AV1 playback is supported with for example the Windows backend.

Tasks:
1. Modify provisioning scripts to download and build the dav1d library
2. Update FFmpeg provisioning scripts to use the dav1d headers and library
3. Update Qt attributions to include the dav1d library
4. Update tests to cover playback of AV1 media files.