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.

[xeraa]

Why does this have to be Cloud specific? Won't the ELASTICSEARCH_URL (or whatever you're using) work locally as well as in any other environment?

[codefromthecrypt]

@xeraa Agree on the main app. This is more about the opentelemetry part, to make README or env.example comments of what to do when you are using Elastic cloud (for APM).