[Asset Store Versioning 1 of 3] Place computed step output versions on the execution plan.
Summary: Adding the step output versions to the execution plan allows them to be accessible at runtime through a SystemStepExecutionContext, which enables them to be used in an asset store's implementation.
Test Plan: Unit tests
Reviewers: sandyryza, yuhan
Reviewed By: sandyryza
Differential Revision: https://dagster.phacility.com/D5081