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/

lib/plugin/instance.rb

@@ -373,6 +373,17 @@ class Plugin::Instance
     assets
   end
 
+  def add_directory_column(column_name, query:, icon: nil)
+    validate_directory_column_name(column_name)
+    directory_column = DirectoryColumn
+      .find_or_create_by(name: column_name, icon: icon, type: DirectoryColumn.types[:plugin]) do |column|
+        column.position = DirectoryColumn.maximum("position") + 1
+        column.enabled = false
+      end
+    DirectoryColumn.add_plugin_directory_column(column_name)
+    DirectoryItem.add_plugin_query(query)
+  end
+
   def delete_extra_automatic_assets(good_paths)
     return unless Dir.exists? auto_generated_path
 
@@ -964,6 +975,11 @@ class Plugin::Instance
 
   private
 
+  def validate_directory_column_name(column_name)
+    match = /^[_a-z]+$/.match(column_name)
+    raise "Invalid directory column name '#{column_name}'. Can only contain a-z and underscores" unless match
+  end
+
   def write_asset(path, contents)
     unless File.exists?(path)
       ensure_directory(path)