Add `config` attributes to `template_fields`

[oboki]

I wish to manage kafka_config attribute as template_fields.

I want to have different bootstrap.servers value depending on the deployment environment, such as develop, test, and production. It would be useful while managing different values in the Variable store for each deployment environment with the same DAG code.

For example, in the ProduceToTopicOperator, I can use:

kafka_config={"bootstrap.servers": "{{ var.value.get('kafka__bootstrap_servers')}}"},

[dylanbstorey]

Thanks for you contribution - it will go out in the next release.