Add wrapper for Python enums in Dagster configs. (#2046)
This adds Enum.from_python_enum which takes a Python enum (i.e.
something inheriting from EnumMeta) and builds a Dagster Enum from it.
Each value is represented by its name on config level but the
corresponding python_value is the enum value itself.
Also adds two tests:
- Test that Enum created from this method can be used in a solid config.
- Test for comprehensiveness: all enum values actually have a corresponding EnumValue.