avoid exposing AssetStoreHandle on OutputDefinition

Summary:
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