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/

db/migrate/20210609133551_add_type_to_directory_columns.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class AddTypeToDirectoryColumns < ActiveRecord::Migration[6.1]
+  def up
+    add_column :directory_columns, :type, :integer, default: 0, null: false
+
+    DB.exec(
+      <<~SQL
+        UPDATE directory_columns
+        SET type = CASE WHEN automatic THEN 0 ELSE 1 END;
+      SQL
+    )
+  end
+
+  def down
+    remove_column :directory_columns, :type, :integer, default: 0, null: false
+  end
+end

db/post_migrate/20210609152431_remove_directory_column_automatic.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class RemoveDirectoryColumnAutomatic < ActiveRecord::Migration[6.1]
+  def up
+    remove_column :directory_columns, :automatic
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end