Expose max_llm_calls to ADK web

[allenli873]

In the runtime config, there is max_llm_calls setting. There is no mechanism to propagate this to RunConfig in the adk web server.

It would be nice to have a way to customize max_llm_calls when using and deploying through ADK web. The simplest thing to do would be to add a new field to get_fast_api_app; more robust solutions might be:

• Add new optional field to run methods, which can be surfaced through the Dev UI/by the user on a case by case basis
• Implement something like https://github.com/google/adk-python/issues/1167