Add AV1 support

[erdnaxe]

AV1 is an open video codec, evolution of VP9/VP8:

• AV1 has SVC built-in, meaning that people with lower bandwidth will be able to enjoy a lower-quality video stream.
• it should work on browsers, except Safari and Firefox on Android (as of 19-05-2021): https://caniuse.com/av1
• it is more CPU intensive than VP8 but it should result in better compression.

If Galène AV1 support works well, we could add AV1 support in Galène-stream using the av1enc module in GStreamer.

Right now rtpav1pay is missing, but we could help and contribute something.

[erdnaxe]

https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/802 has been merged.