grokkability improvements for partitions
- Added some mypy types, including making Partitions generic.
- schedule_partition_range now returns a list of partitions instead of a function that returns a list of partitions.
- PartitionParams -> Partitions. I don't 100% love Partitions as a replacement, but IMO it gets the meaning across better. PartitionParams makes me expect a dumb struct of parameters to some fancy function.
Test Plan: bk
Reviewers: dgibson, rexledesma
Reviewed By: dgibson, rexledesma
Differential Revision: https://dagster.phacility.com/D8612