Make attached sensor transform changeable during runtime

[fpasch]

Currently carla_spectator_camera provides the possibility to spawn an vehicle-attached camera sensor at a specific pose and then modify the pose during runtime.

We should move that functionality to carla_ros_bridge and make it available to all (camera) sensors.

The idea is to have a ros-only attribute per sensor, e.g. allow_transform. If true the carla_ros_bridge provides a topic to update the pose.