DynamicOutput DSL map approach
Summary: Alternative to D5613, this demonstrates a map method for unpacking dynamic outputs in the DSL.
Test Plan: added and updated tests
Reviewers: sashank, catherinewu, schrockn, sandyryza, prha
Reviewed By: schrockn
Subscribers: prha
Differential Revision: https://dagster.phacility.com/D5721