Design sketch for quota support at the IFilesystem layer

This notes where changes are necessary support some volume size features.  Maximum volume size is a good approximation of the idea of quotas without being overly backend specific (for example, EBS volumes can't have quotas, they just have a size).
This only checks the `size` argument. I suspect in the future, we will have more arguments to pass here. So this test should either test that all arguments are actually passed (I guess this would need a fake of some sort?) or it should be explicit about what it tests.