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

lib/discourse_plugin_registry.rb

@@ -24,6 +24,29 @@ class DiscoursePluginRegistry
     end
   end
 
+  # Create a new register (see `define_register`) with some additions:
+  #   - Register is created in a class variable using the specified name/type
+  #   - Defines singleton method to access the register
+  #   - Defines instance method as a shortcut to the singleton method
+  #   - Automatically deletes the register on ::clear!
+  def self.define_filtered_register(register_name)
+    define_register(register_name, Array)
+
+    singleton_class.alias_method :"_raw_#{register_name}", :"#{register_name}"
+
+    define_singleton_method(register_name) do
+      unfiltered = public_send(:"_raw_#{register_name}")
+      unfiltered
+        .filter { |v| v[:plugin].enabled? }
+        .map { |v| v[:value] }
+        .uniq
+    end
+
+    define_singleton_method("register_#{register_name.to_s.singularize}") do |value, plugin|
+      public_send(:"_raw_#{register_name}") << { plugin: plugin, value: value }
+    end
+  end
+
   define_register :javascripts, Set
   define_register :auth_providers, Set
   define_register :service_workers, Set
@@ -44,6 +67,14 @@ class DiscoursePluginRegistry
   define_register :vendored_pretty_text, Set
   define_register :vendored_core_pretty_text, Set
 
+  define_filtered_register :staff_user_custom_fields
+  define_filtered_register :public_user_custom_fields
+
+  define_filtered_register :self_editable_user_custom_fields
+  define_filtered_register :staff_editable_user_custom_fields
+
+  define_filtered_register :editable_group_custom_fields
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end