Add support for display_name in sessions

[dylan-apex]

When you make a POST request to /apps/{app_name}/users/{user}/sessions, there's no support for display_name with VertexAI. _create_session in adk_web_server.py doesn't pass display_name, even if it's included in the request body. The rest of the VertexAI session creation process accepts **kwargs, so we could also pass **kwargs in the signature