Enum fields should be generated as Field::Select instead of Field::String

Created by: betelgeuse

• What were you trying to do? Run generator for a ActiveRecord model with an enum column.
• What did you end up with (logs, or, even better, example apps are great!)? Dashboard with Field::String
• What versions are you running?
  • Rails: 6.0
  • administrate: master

The generated field should be like this

enum_attribute: Field::Select.with_options(collection: Model.enum_attributes.keys)

With Field::String submitting to the update action with invalid value results in a Server Error of ArgumentError in Admin::ModelController#update