Set up backend content manager

[DenisaCG]

This PR sets up the backend manager. The logic to transition to one provider independent manager was added, by modifying the already existing JupyterDrivesManager class. To achieve this, the architecture for the entrypoints was modified, as we only need to register one manager for all providers now.

The listing drives functionality was updated to have the provider independent set up.

The mount_drive functionality was added in the backend and its corresponding frontend request. It is meant to be used when first accessing a drive, in order to create the content manager/store for it in the backend. Consequently, the mounted property was added to IDriveInfo interface to signal whether a content manager was already set up in the backend for the specific drive.

Also, switched to the use of the obstore package for the backend.

[github-actions]

👈 Launch a Binder on branch DenisaCG/jupyter-drives/contentsManager

[afshin]

Thank you!