DEV: Add framework for filtered plugin registers (#9763)

* DEV: Add framework for filtered plugin registers

Plugins often need to add values to a list, and we need to filter those lists at runtime to ignore values from disabled plugins. This commit provides a re-usable way to do that, which should make it easier to add new registers in future, and also reduce repeated code.

Follow-up commits will migrate existing registers to use this new system

* DEV: Migrate user and group custom field APIs to plugin registry

This gives us a consistent system for checking plugin enabled state, so we are repeating less logic. API changes are backwards compatible

app/controllers/groups_controller.rb

@@ -545,7 +545,7 @@ class GroupsController < ApplicationController
             :publish_read_state
           ])
 
-          custom_fields = Group.editable_group_custom_fields
+          custom_fields = DiscoursePluginRegistry.editable_group_custom_fields
           default_params << { custom_fields: custom_fields } unless custom_fields.blank?
         end