[FEATURE REQUEST] Expose available `FilterDesc` info through Python bindings

[nrusch]

As a developer building tools that use OIIO, it would be very helpful to be able to enumerate the available filters and their parameters (name, default width, whether it's fixed-size, etc.) using the Python bindings, in order to facilitate argument validation, construction of dynamic UIs, etc.

My first thought on how to accomplish this would be to wrap Filter(1/2)D and FilterDesc, and expose access to the available instances using a similar (but perhaps slightly more "pythonic") pattern to the C++ API.

This certainly isn't high-priority at all, and I may be able to take a run at it at some point, but I at least wanted to get it logged.