Add example using autogen-core and FastAPI for handoff multi-agent design pattern with streaming and UI (#6391)

## Why are these changes needed?

This PR adds an example which demonstrates how to build a streaming chat
API with multi-turn conversation history and a simple web UI for handoff
multi-agent design pattern.

---------

Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>