Dropbox support

For now the only provider outside of filesystem is Box. Would like to have Dropbox as an option as well. Willing to work on it.

Start by locating the existing Box and filesystem provider implementations and trace how providers are exposed as options. Define the Dropbox integration's scope from those existing providers, then verify that Dropbox appears as a usable option and that the relevant provider behavior is covered by the repository's tests.

Written by the indexing model from the issue text.