DEV: Plugin API to allow creation of directory columns with item query (#13402)

The first thing we needed here was an enum rather than a boolean to determine how a directory_column was created. Now we have `automatic`, `user_field` and `plugin` directory columns.

This plugin API is assuming that the plugin has added a migration to a column to the `directory_items` table.

This was created to be initially used by discourse-solved. PR with API usage - https://github.com/discourse/discourse-solved/pull/137/

spec/requests/admin/user_fields_controller_spec.rb

@@ -130,7 +130,7 @@ describe Admin::UserFieldsController do
         DirectoryColumn.create(
           user_field_id: user_field.id,
           enabled: false,
-          automatic: false,
+          type: DirectoryColumn.types[:user_field],
           position: next_position
         )
         expect {