Support preview configuration for dynamic schedule configs
If a ScheduleDefinition uses a dynamic config (environment_config_fn), then Dagit would not properly load the config. This diff updates the gql resolvers to evaluation the config function if it exists to support the view configuration feature.
Caveat is that environment_config_fn can't have any side effects.
Test Plan: unit
Reviewers: #ft, alangenfeld
Reviewed By: #ft, alangenfeld
Differential Revision: https://dagster.phacility.com/D1404