Add on_tool_result()/on_tool_request() callbacks

[cpsievert]

Addresses #33

Since taking a first pass at this, I've realized it'd be better if on_tool_result()/on_tool_request() expected a function that returns a string. And, that string would then be yielded by .chat() and friends (when echo="text"). This way, adding custom tool UI in Shiny or at the console would be fairly trivial, just do something like:

@chat.on_tool_request
def _(request):
    return f"Calling tool {request.name}..."

@chat.on_tool_result
def _(result):
    return f"Got tool result: {result.value}"

And, to make it easier to create good Shiny UI (i.e., put in a card, maybe with a title and progress indicator), we could perhaps have <shiny-chat-tool-request> and <shiny-chat-tool-result> component helpers?

cc @gadenbuie

[cpsievert]

#69 supersedes this