Constant-Folding Registry

Authoring custom constant-folding for operators via the `register` method in `onnxscript.optimizer._constant_folding` feels quite convenient (see #2506 for example) - are there any plans to properly expose this to the user to make constant folding customizable? I mean, it is already possible to simply `from onnxscript.optimizer._constant_folding import register`, but this is importing from a protected memeber (at least the linter complains), so I guess it is not meant to be used this way, at least for now?