Support authentication with refresh token + app key

[edgarrmondragon]

Using a refresh token and app key to get a new short-lived token can be done before instantiating DropboxDriveFileSystem, but it'd be nice if the filesystem implementation handled it completely for the developer.

It would enable backend applications to simply store a refresh token and app key, and pass them to fsspec.filesystem(<protocol>, ...) in a generic way.

This is already supported by the Dropbox Python SDK.