Support optional deps or separate arti-* packages for connectors

[JacobHayes]

Eg: we'll be adding a gcsfs dep soon for a GCSFile(Storage). This shouldn't be required for all users.

We do some dynamic submodule imports in a few spots in the code base that might cause some trouble with optional deps. If we do keep these contrib connectors in-tree (as opposed to moving to arti-* packages that extend the namespace), we'll need to make sure those skip gracefully. Perhaps we add a custom import error class that we can raise in our "optional" modules (and then catch that in import_submodules).