[dagit] Add id to Asset

Summary: Give the Asset type an id field to avoid Apollo cache issues. Not sure this is the most ideal ID value here (e.g. AssetKey(['table_info'])), so let me know if I should change it.

Test Plan: Buildkite, load Dagit and sanity check global search behavior and Assets page behavior.

Reviewers: prha, sandyryza

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D7222