-Allows displaying custom options for specific file format variants
-Added support for scene format import to retrieve custom options
This PR is necessary for #54886 to be implemented properly.
* New plugin system to control the whole import workflow
* Can add options and run code at every import step (general, per node, mesh, animation, material etc.)
This constitutes a first version of these plugins. The ability to interact with the import preview dialog will likely be added later on.