Feature request

Feature description

Currently nav2_bringup/bringup_launch.py is perfect for launching the navigation of a robot. With minimal modifications to the parameter file I can run on our PMB2 robot.

But when setting slam:=True, bringup_launch.py includes slam_launch.py which includes slam_toolbox/online_sync_launch.py without passing a parameter file. It always loads the one included on the package.

To change such params, I have to create a separate online_sync_launch.py file, and therefore modify bringup_launch.py and slam_launch.py.