Support Tool Calls that can report sub-steps or sub-Tasks

[antonioalegria]

Use case is I have tool calls that can take a few minutes and have a few large steps - think for example scraping a web site with multiple pages with some post-processing afterwards (e.g. extracting information). It would be good to let the user know we're at step 2/5, etc...

So it would be interesting to have a convention, to signal sub-steps in a tool call like we do with background tasks, and have it show in the streamlit app.

See discussion here: #97