chatbot-rag-app: re-introduce ES cloud configuration

[codefromthecrypt]

When we refactored chatbot-rag-app, we started with local ENV. That said, many folks use ES cloud. We should add support in code and env.example for ES cloud connections, including opentelemetry setup.

Ideally, this can use canonical ENV variables used in other elastic tools, without us having to hard code if statements about cloud vs not. That said, I'm not sure if elasticsearch-py supports that at the moment.