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).
Sounds reasonable.
This pull request relies on https://github.com/OpenImageIO/oiio/pull/939, which we'd like to get merged first as well.  It's probably possible to get the nuke plugins working without that fix, but it has not been tested.