Add: simple st20tx plugin for gstreamer (#1015)

Add simple st20tx plugin for GStreamer to enable ST2110-20 TX
stream creation

- Implement a basic st20tx sink plugin for GStreamer.
- Add structures for raw video parsing to support future raw
video parsing from filesrc.
- Add support for partial frames from GStreamer as long as they
add up to the correct frame size.
- Support "waiting" for MTL library by transitioning the plugin to the
PLAYING state when MTL framebuffers are unavailable.

Known Issues:
- Small corruption (~73 pixels) in the top left corner of the stream during
v210 -> internal conversion device to yuv422_10le -> transmission 
receive -> yuv422_10le -> internal conversion device to v210.