This renames AssetStore to ObjectManager and has it inherit from InputManager and OutputManager.
ObjectManagerDefinition inherits from IInputManagerDefinition and IOutputManagerDefinition.
This diff maintains compatibility with the prior AssetStore naming. That will allow us to update all the callsites in a followup diff.
Depends on D5427.
Test Plan: bk
Reviewers: schrockn, alangenfeld, yuhan
Reviewed By: schrockn
Differential Revision: https://dagster.phacility.com/D5437