Change the time source for ROS2 bags and allow for selecting any time

[EmmanuelMess]

Plotjuggler should allow the user to select either the "received timestamp", "sent timestamp" or "rosgraph_msgs/Clock" message topic (if available).

Because of the fact that the send timestamp can be in any time system (usually the user's unix timestamp, see ros2/rosbag2#2023 ) it should not be the default.