fix: ros2 gstreamer timestamps

[drwnz]

Addresses #84
When use_gst_timestamps was is set to true, timestamps converted from the gstreamer buffer timestamps would be incorrect.
This is a ROS2 specific issue, caused by no constructor for rclcpp::Time which takes the time parameter in seconds as a float type.

This PR:

• Adds use_gst_timestamps to the readme
• Uses nanoseconds instead of seconds to calculate the offset timestamps from the gstreamer buffer timestamps

It has been tested on Galactic.
@wep21

[wep21]

I confirmed the changes modified the timestamp in header even when use_gst_timestamps is true.
before:

stamp:
  sec: 1
  nanosec: 655143410
frame_id: camera_frame

after:

stamp:
  sec: 1655143667
  nanosec: 434399126
frame_id: camera_frame

[wep21]

LGTM

[wep21]

@jbohren @clydemcqueen the changes look good to me. What do you think?

[clydemcqueen]

LGTM. I tested this on Galactic and it works as expected. Thanks @drwnz for finding & fixing and @wep21 for moving this forward!

I noticed a few unrelated bugs in the README:

• the build badge references clydemcqueen/gscam, it should reference ros-drivers/gscam
• the GStreamer version for Rolling should be 1.20, not 1.18
• Humble should be added to the list of supported versions

/Clyde

[wep21]

@clydemcqueen Thank you for confirming it. Could you create another PR to fix README?

[wep21]

@drwnz Thank you for fixing the bug.

[clydemcqueen]

I submitted a PR to fix the README.