Schema support for color transformations

The only color transformation support currently modeled in OTIO right now is CDL info. This is stored in a metadata dictionary.
Color transformations should be modeled as `otio.schema.Effect`.

This might include:
- ASC CDL (Saturation and slope, offset, power)
- CUBE LUTs
- (your suggestion here!)

Start by reading the existing CDL metadata representation and the otio.schema.Effect entry point. Define what color transformations should be modeled as Effects, including ASC CDL and CUBE LUTs, and confirm the schema and interchange behavior with project maintainers before implementation.

Written by the indexing model from the issue text.