code: def __AddAdditionalPropertyType(self, name, property_schema):
          
          new_type_name = 'AdditionalProperty'
          property_schema = dict(property_schema)
          # We drop the description here on purpose, so the resulting
          # messages are less repetitive.
          property_schema.pop('description', None)
          description = 'An additional property for a %s object.' % name
          schema = {
              'id': new_type_name,
              'type': 'object',
              'description': description,
              'properties': {
                  'key': {
                      'type': 'string',
                      'description': 'Name of the additional property.',
                  },
                  'value': property_schema,
              },
          }
          self.AddDescriptorFromSchema(new_type_name, schema)
          return new_type_name
docs: Add a new nested AdditionalProperty message.