[dagit] Make RunsSearchSpaceQuery lazy
Summary: It turns out that RunsSearchSpaceQuery is pretty expensive. Make it lazy, waiting for the user to focus the input.
Test Plan: View Runs in Dagit, focus the filter input. Verify that it performs the query at this point and no sooner.
Reviewers: bengotow, prha
Reviewed By: bengotow, prha
Differential Revision: https://dagster.phacility.com/D7685