[dagit] Add Permissions query
Summary: Create a PermissionsProvider that queries for permissions via GraphQL, as arranged in https://dagster.phacility.com/D9122.
Test Plan: Load Dagit, verify that permissions are queried and used properly.
Reviewers: dgibson
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D9185