v4l2codecs: add AV1 decoder
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/2305
As the new AV1 V4L2 uAPI progresses, add a suitable decoder element to proof-test it. This MR shall remain a draft until the actual uAPI is upstreamed in the Linux kernel.
This MR is based on !1567 (merged)