Add environment variable to control category filtering

OCIO color spaces contain a categories attribute that allows config authors and applications to filter the list of color spaces shown in a given menu based on its function (e.g. working spaces, textures). The ColorSpaceMenuHelper class provides app developers an easy way to use this functionality.

There is currently a OCIO_USER_CATEGORIES_ENVVAR that allows users to specify categories to use with application menus. It would be nice if there was one or more additional env. vars to control menu filtering. For example, it might be useful to be able to turn off filtering being done by an application.

Discussed at the 2025-09-29 TSC meeting.

首先定位 ColorSpaceMenuHelper 以及对 OCIO_USER_CATEGORIES_ENVVAR 的处理。查看 2025-09-29 的 TSC 讨论，以确定所需的其他环境变量行为，包括是否可以禁用应用程序过滤。完成标准是实现已达成一致的行为，并提供相应的覆盖率。

由索引模型根据 Issue 内容生成。