Adds initial support for File Handler registration with the python API
for design task #68935. File Handlers will allow developers to associate
additional UI behavior and capability to operators traditionally used
only within the file browser.
The initial commit should have no user visible changes, but will serve
as the foundation for providing file drag & drop capabilities to
operators that can manage files (#111242).
See the PR for an example of python usage.
See design task #68935 for remaining work to be explored in the future.
Pull Request: https://projects.blender.org/blender/blender/pulls/112466