gstdecode: add AV1

Tested with:

./vlc -I "dummy" -vvvvvvvvvvvvvvvvv --codec gstdecode elecard-trimmed.ivf 2>&1 | grep gst
[00007f144ca684f8] main generic debug: looking for video decoder module matching "gstdecode": 14 candidates
[00007f1424035c20] gstdecode decoder debug: Using decodebin? yes 
[00007f1424035c20] gstdecode decoder debug: Using vlc pool? no
[00007f1424035c20] gstdecode decoder debug: appsrc seeking to 0
[00007f144ca684f8] main generic debug: using video decoder module "gstdecode"
[00007f1424035c20] gstdecode decoder debug: linking the decoder with the vsink
[00007f1424035c20] gstdecode decoder: got new caps video/x-raw, format=(string)I420, width=(int)1920, height=(int)1080, interlace-mode=(string)progressive, multiview-mode=(string)mono, multiview-flags=(GstVideoMultiviewFlagsSet)0:ffffffff:/right-view-first/left-flipped/left-flopped/right-flipped/right-flopped/half-aspect/mixed-mono, pixel-aspect-ratio=(fraction)1/1, chroma-site=(string)mpeg2, colorimetry=(string)bt709, framerate=(fraction)25/1
[00007f1424035c20] gstdecode decoder debug: Ensuring the decoder queue is empty
[00007f1424035c20] gstdecode decoder debug: Ensured the decoder queue is empty
[00007f1424035c20] gstdecode decoder debug: Pipeline is prerolled