Add pillow as an optional dependency

[hasansezertasan]

Some validators and processors require pillow to be used. Isn't it good to add pillow as an optional dependency? So the developers can install the sqlalchemy-file with the pip install sqlalchemy-file[image] command without worrying about other dependencies and their versions.

I don't remember why but I had to install fasteners to run my application and I'm not sure if I should pin the version or not.

[hasansezertasan]

I just double-checked: the fasteners package is required by LocalStorageDriver.