Add offset-timestamp param

[LihanChen2004]

I have added a new parameter offset_timestamp to the joint_state_publisher.

Here’s an explanation of why I needed this parameter, and I believe other developers might also find it useful.

My robot setup is as follows: the gimbal_yaw and gimbal_pitch are actuable, and their attitude is obtained via the IMU sensor on the embedded development board. The data is then transmitted to the NUC via UART as a JointState type. I use the joint_state_publisher and robot_state_publisher to publish the full vehicle’s TF data.

One subtle issue is that there is a delay in the data transfer from the embedded development board to the NUC (approximately 0.008 seconds in our system based on testing). Although this delay is small, the real-time nature of our development work makes it impactful. It causes an apparent lag in the transformation of the gimbal_pitch sensor into chassis (or other fixed world coordinate frame). After adding the offset_timestamp, the issue was significantly mitigated.