Add Nuke txReader/txWriter plugin sources and build files

Add source and build code for Nuke plugins to read and write .tx files using OIIO. These were originally written at Luma against a much older version of OIIO, but have been updated to work with the current API.

Reader currently supports reading of a single mip level at the file's base resolution (limited to 4 channels at present).

Writer supports a subset of the `maketx` options, and is also currently limited to 4 channels due to what seems to be an issue with OIIO storing custom channel names (support for extra named channels is currently parked on another internal branch until it can be used).
Understood. You may still want to move this into the first call to engine() or request() since I think (I'm not 100% sure of this) that open() is called prior to validate() and would cause the texture to be opened even if Nuke is only requesting header data but not reading pixels. Disregard if you find that this is not the case and open() is only called before engine().