DEV: Refactor checkpoint logic from FileContentsManager. - Add a `CheckpointManager` base class and infrastructure for creating a `checkpoint_manager` instance attribute on `ContentsManager`. - Provide default implementations of `delete` and `rename` in the base `ContentsManager` class. `ContentsManager` subclasses are now required to implement `delete_file` and `rename_file`. These methods no longer need to manage checkpoints. - Move checkpoint-related functionality from `FileContentsManager` to a dedicated `FileCheckpointManager` subclass. - Move shared filesystem interaction logic into `FileManagerMixin` used by both `FileContentsManager` and `FileCheckpointManager`. - Minor tweaks to ContentsManager tests to get methods from the right object. The purpose of this change is to provide an API for users to replace just the checkpoint logic associated with a particular `ContentsManager`. In particular, this change makes it possible to easily support remote storage of checkpoints while otherwise retaining normal filesystem interactions.