Split mongo assets cherry

Asset support in split
LMS-2876

Kind of, because really using in split requires the opaque url refactoring and CourseLocator.make_asset_key, this actually doesn't do anything with split, but it gets close.

@nasthagiri @cahrens @dianakhuang please review
May need to either change the docstring to clarify that this method should only be used by code which only makes sense in a heterogenous ms env which uses mixed to route or may need to implement a base version of this which verifies that self is of type store_type and let the mixed one override that default impl.