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

spec/requests/groups_controller_spec.rb

@@ -620,7 +620,7 @@ describe GroupsController do
       end
 
       after do
-        Group.plugin_editable_group_custom_fields.clear
+        DiscoursePluginRegistry.reset!
       end
 
       it "only updates allowed user fields" do
@@ -634,7 +634,7 @@ describe GroupsController do
       end
 
       it "is secure when there are no registered editable fields" do
-        Group.plugin_editable_group_custom_fields.clear
+        DiscoursePluginRegistry.reset!
         put "/groups/#{@group.id}.json", params: { group: { custom_fields: { test: :hello1, test2: :hello2 } } }
 
         @group.reload