avoid exposing AssetStoreHandle on OutputDefinition
OutputDefinition is a public API, so any members of it are public as well. I think we should avoid
exposing asset_store_handle to users, to avoid another concept that they need to deal with.
Test Plan: bk
Reviewers: yuhan, cdecarolis
Reviewed By: yuhan
Differential Revision: https://dagster.phacility.com/D5045