`on_exit_shutdown` is not evaluated correctly

[azeey]

Environment

• OS Version: Ubuntu 22.04
• Source or binary build? 2dca3580f7ac27f954fcdffbcff956aba963076f

Description

• Expected behavior: Setting on_exit_shutdown launch argument to false to work properly
• Actual behavior: on_exit_shutdown is a str when evaluated in the if statement below: https://github.com/gazebosim/ros_gz/blob/2dca3580f7ac27f954fcdffbcff956aba963076f/ros_gz_sim/launch/gz_sim.launch.py.in#L58

This means setting it to 'true' or 'false' will always be treated as True. Perhaps we can use IfCondition and call evaluate here instead?