Write OpenFX plug-in for FileTransform

Issue #1117 requests OpenFX plug-ins for OCIO and Michael has PR #1371 in progress that is almost complete to implement a ColorSpaceTransform and DisplayViewTransform plug-in, which are the key ones. It would also be great to have a few more OpenFX plug-ins, for example for FileTransform. This would allow people to apply stand-alone LUTs in any of the formats OCIO supports, including the Academy/ASC Common LUT Format (CLF). The implementation should include an "invert" toggle button, similar to Michael's work.

It would also be great to have OpenFX plug-ins for LookTransform and NamedTransform, although these could also be in separate PRs.

Start with issue #1117 and PR #1371, which provide the existing OpenFX plug-in context for OpenColorIO. Determine how the FileTransform plug-in should fit alongside the ColorSpaceTransform and DisplayViewTransform work, including the invert toggle. Done means a FileTransform OpenFX plug-in can apply supported standalone LUT formats, including CLF; LookTransform and NamedTransform are separate follow-up scope.

Written by the indexing model from the issue text.