[FEATURE REQUEST] Improve Python ColorConfig support (in IBA, etc.)

As a developer building Python tools using OIIO, it would be great to be able to manage and use `ColorConfig` instances (including the default instance) using the OIIO Python bindings. To that end, there are a couple of extensions to the current color config support that would be very useful.

**Support passing `ColorConfig` instances to Python IBA functions**

OIIO exposes the `ColorConfig` class through the Python bindings, but the color-related `ImageBufAlgo` functions that take a `colorconfig` argument only accept a string value. It would be great to be able to pass an actual `ColorConfig` instance to these functions, so that the host code using OIIO could create and manage color configs itself.

**Support managing the default `ColorConfig` instance using the OIIO API (including from Python)**

OIIO partially piggybacks on OCIO's concept of a global config instance by looking up `OCIO::GetCurrentConfig()` to underpin any `ColorConfig` that is instantiated with an empty string.

However, when using OIIO in an environment where the OCIO libraries/bindings OIIO uses may not be available at runtime (e.g. due to namespace differences, etc), there is no way to **set** the global OCIO config in order to change the behavior of future default-constructed `ColorConfig` instances. The only way to influence them at all is to set the `OCIO` environment variable, but since OCIO caches the first config created from the environment when you call `OCIO::GetCurrentConfig()`, the OCIO config used for default-constructed `ColorConfig` cannot be swapped out after its first use.

I think these pitfalls could be avoided if OIIO added a basic API for getting/setting the default `ColorConfig` instance (maybe via static getter/setter methods?).

I would say the first request would definitely be more important/impactful than the second, but both would be much appreciated.

Start by locating the Python bindings for ColorConfig and the ImageBufAlgo functions that accept a colorconfig argument, then review how default-constructed ColorConfig instances use OCIO::GetCurrentConfig(). Done means Python IBA calls accept ColorConfig instances and the OIIO API provides a way to get and set the default ColorConfig, including from Python.

Written by the indexing model from the issue text.