Add support for extra_fields query parameter

Api now supports optional fields.
These are included with the extra_fields query param.

Syntax: extra_fields[type]=field1,field2

app/controllers/api/v1/base_controller.rb

@@ -106,7 +106,9 @@ module Api
       end
 
       def json_api_error(message, **options)
-        { errors: [{ detail: message }] }.merge(options)
+        error_options = options.delete(:error_options) || {}
+
+        { errors: [{ detail: message }.merge(error_options)] }.merge(options)
       end
 
       def json_api_invalid(message, errors)

app/controllers/concerns/extra_fields.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+# To be included in api controllers for handeling query params
+module ExtraFields
+  extend ActiveSupport::Concern
+
+  def invalid_query_param(name, status, msg)
+    render status: status, json: json_api_error(msg, error_options:
+      {
+        title: I18n.t("api.query_param.error.title"),
+        source: { parameter: name },
+        status: status,
+        code: Rack::Utils::SYMBOL_TO_STATUS_CODE[status]
+      })
+  end
+
+  def extra_fields(type, available_fields)
+    fields = params.dig(:extra_fields, type)&.split(',')&.compact&.map(&:to_sym)
+    return [] if fields.blank?
+
+    unknown_fields = fields - available_fields
+
+    if unknown_fields.present?
+      invalid_query_param(
+        "extra_fields[#{type}]", :unprocessable_entity,
+        I18n.t("api.query_param.error.extra_fields", fields: unknown_fields.join(', '))
+      )
+    end
+
+    fields
+  end
+end