Export frame ITUT-T35 Metadata

Based on a patch by Renato Cassaca. Fixes #291 (closed)

https://0x0.st/zOVO.ivf (custom made by re-encoding a h264 sample with closed captions and inserting the metadata OBU using ffmpeg's av1_metadata bsf).

Soname is bumped since this adds new fields to Dav1dPicture, and to prevent having to do it again next time we add new metadata types, i added a few reserved fields like we did with Dav1dSettings.