code: def convert_descriptor(self, descriptor):
          
  
          # Fields
          fields = []
          fallbacks = []
          schema = tableschema.Schema(descriptor)
          for index, field in enumerate(schema.fields):
              converted_type = self.convert_type(field.type)
              if not converted_type:
                  converted_type = 'STRING'
                  fallbacks.append(index)
              mode = 'NULLABLE'
              if field.required:
                  mode = 'REQUIRED'
              fields.append({
                  'name': _slugify_field_name(field.name),
                  'type': converted_type,
                  'mode': mode,
              })
  
          # Descriptor
          converted_descriptor = {
              'fields': fields,
          }
  
          return (converted_descriptor, fallbacks)
docs: Convert descriptor to BigQuery