Rclcpp: add toSec() function to the rclcpp::Time Class

[hakuturu583]

Feature request

In ROS1, we can get duration and time in seconds.
Via toSec() method. (ros::TimeBase< T, D > Class Template Reference)
But now, I think there is no API to get the time and duration in Seconds.
So, I want to add toSec() method to the rclcpp::Time Class

Feature description

toSec() method returns the rclcpp::Time and Duration data in Seconds.

Implementation considerations

[dirk-thomas]

That functionality already exists since #526. Please note the docblock about the potential precision loss.

[hakuturu583]

Thanks!