add log_setup_fnc for cli.run_app

[longcw]

close https://github.com/livekit/agents/issues/4339

for example

def log_setup(logger: logging.Logger, log_level: int | str):
    logger.setLevel(log_level)
    handler = logging.StreamHandler(sys.stdout)
    handler.setFormatter(logging.Formatter("%(asctime)s - %(levelname)s - %(message)s"))
    logger.addHandler(handler)


if __name__ == "__main__":
    cli.run_app(server, log_setup_fnc=log_setup)