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

[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

[pablobm]

This might be a matter of changing this mapping:

administrate/lib/generators/administrate/dashboard/dashboard_generator.rb

Lines 3 to 16 in 15c49a6

	module Administrate
	module Generators
	class DashboardGenerator < Rails::Generators::NamedBase
	ATTRIBUTE_TYPE_MAPPING = {
	boolean: "Field::Boolean",
	date: "Field::Date",
	datetime: "Field::DateTime",
	enum: "Field::String",
	float: "Field::Number",
	integer: "Field::Number",
	time: "Field::Time",
	text: "Field::Text",
	string: "Field::String",
	}

Not 100% sure though. @betelgeuse, do you think you could try a PR?