Returns custom attributes belonging to the authenticated account, joined with their group. Optional filters: custom_attr_group_uuid (account-scoped UUID, returns 422 if unknown), custom_attr_key (attribute key). Optional order_by map of column => direction for sorting; valid columns are custom_attr_display_order, custom_attr_name, custom_attr_key, custom_attr_type, custom_attr_group_name. Default order: custom_attr_display_order ASC, custom_attr_name ASC.