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

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.

The requested entry points are the Python bindings for Filter(1/2)D and FilterDesc, exposed similarly to the C++ API. Start by reading those APIs and determine how filter names, default widths, and fixed-size status are represented; done means Python callers can enumerate filters and inspect those parameters for validation and dynamic UI use.

Written by the indexing model from the issue text.