Ability to specify a lucid icon on a step

[Allaoua9]

trafficstars

Is your feature request related to a problem? Please describe. An agent can call multiple tools (doc search, web search, image gen, etc). In our implementation, when a tool is called, we display a step. We find that displaying a specific icon when a tool is called produces better UX.

I have created a pull request of our implementation here : https://github.com/Chainlit/chainlit/pull/2561