This is a special base class for Storages which are using ids instead of
paths in their APIs, like Box or Google Drive.
This commit makes Box derived from IdStorage.
And used in it BoxResolveIdRequest.
TODO: make some generic ResolveIdRequest and ListDirectoryRequest for
id-based storages. It's really similar, I just had to change a few
details in GoogleDrive ListDirectory and ResolveId requests.