Add a full parser to libavcodec for AV_CODEC_ID_JPEGXL. It finds the
end of the stream in order to packetize the codec, and it looks at
the headers to set preliminary information like dimensions and pixel
format.

Note that much of this code is duplicated from avformat/jpegxl_probe.c,
but that code will be removed and call this instead in the next commit.
Signed-off-by: Leo Izen <leo.izen@gmail.com>