Add a parameter to control whether the gripper opens when stretch driver shuts down

[hello-amal]

Currently, if we terminate the driver in stretch_ros2 while the gripper is closed, the gripper opens. This can be desired behavior -- e.g., if the gripper is holding onto a person -- but can also be undesired -- e.g., if the gripper is holding onto a tray or mug.

For our upcoming use-case where the gripper will be holding onto a tray/object, it would be really helpful to have a flag that we can specify in stretch_user_params to have it keep the gripper closed even after we terminate the stretch driver. The reason for this is that sometimes, ROS2 communication stops and we have to restart ROS nodes. If that happens while the robot is holding onto an object, we don't want to drop the object.