Merge AJA audio/video source/sink plugin
Original location is https://github.com/centricular/gst-aja
Previous discussion about this is at https://discourse.gstreamer.org/t/aja-capture-playout-plugin-move-it-upstream-or-not/75.
This now builds the AJA NTV2 SDK as a meson subproject from github, or optionally lets the user specify a location for where to find the SDK (the one on GitHub has fewer features than the "real" one).