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/users_controller_spec.rb

@@ -1851,8 +1851,7 @@ describe UsersController do
             end
 
             after do
-              User.plugin_editable_user_custom_fields.clear
-              User.plugin_staff_editable_user_custom_fields.clear
+              DiscoursePluginRegistry.reset!
             end
 
             it "only updates allowed user fields" do
@@ -1903,8 +1902,7 @@ describe UsersController do
             end
 
             it "is secure when there are no registered editable fields" do
-              User.plugin_editable_user_custom_fields.clear
-              User.plugin_staff_editable_user_custom_fields.clear
+              DiscoursePluginRegistry.reset!
               put "/u/#{user.username}.json", params: { custom_fields: { test1: :hello1, test2: :hello2, test3: :hello3 } }
               expect(response.status).to eq(200)
               expect(user.custom_fields["test1"]).to be_blank