[dagit] Extract AppCache to top level, consolidate TestProvider
Summary: Move AppCache out of AppProvider so that it can be more flexible about the supplied schema. Also consolidate providers used for tests and storybook, to make it a little easier to set up specs without a ton of requirements.
Test Plan: Run Dagit to sanity check that everything is working. Jest, lint, ts.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D7889