Allow parsing a LUT from a memory buffer rather than a file

OCIO's LUT file parsing is set up to work from files on disk. It would be very helpful to be able to have a function that would take a memory buffer and use the LUT parsers to turn it into a GroupTransform.

There is a way to do this already using the recent ConfigIOProxy class. However, it takes a fair amount of code and a fair amount of OCIO expertise to write it. This task is to write that code and turn it into a parseFromBuffer method on the GroupTransform class.

The unit tests for ConfigIOProxy in Config_tests.cpp provide some good sample code that could be used as a starting point.

Start with the ConfigIOProxy unit tests in Config_tests.cpp, then trace how GroupTransform currently uses the LUT parsers for files. Implement and expose GroupTransform::parseFromBuffer using the existing buffer-based approach, and add coverage showing that a memory-buffer LUT produces the expected GroupTransform.

Written by the indexing model from the issue text.