add summary table for asset keys
This is separate from the asset key structure, which we can maintain using the same schema.
Will start reading/writing to the new summary table.
Users will need to run a migration script in order for all historical asset keys to be visible in the index page.
Test Plan: back compat test
Reviewers: max, alangenfeld, schrockn
Reviewed By: max, alangenfeld
Differential Revision: https://dagster.phacility.com/D4764