Convert a column to a bigquery schema object.

def column_to_bq_schema(self):
        """Convert a column to a bigquery schema object.
        """
        kwargs = {}
        if len(self.fields) > 0:
            fields = [field.column_to_bq_schema() for field in self.fields]
            kwargs = {"fields": fields}

        return google.cloud.bigquery.SchemaField(self.name, self.dtype,
                                                 self.mode, **kwargs)