v4l2codecs: add wrappers for alpha decode

v4l2codecs: add wrappers for alpha decode

codecalpha is a new plugin introduced to support VP8/VP9 alpha as
defined in the WebM and Matroska specifications. It splits the stream
into two streams, one for the alpha and one for the actual content,
then it decodes them separately with vpxdec and finally combine the
results as A420 or AV12 (i.e. YUV + an extra alpha plane).

The workflow above is setup by means of a bin, gstcodecalphabin.

This patch simulates the same workflow into the v4l2codecs namespace,
thus using the new v4l2 stateless decoders for hardware acceleration.

This is so we can register the new alpha decode elements only if the
hardware produces formats we support, i.e. I420 or NV12 for now.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2272>