Add create_timer interface to Node

[jonselling]

Feature request

Feature description

Add an rclcpp::Node interface for the creation of ROS time timers. This would allow easier creation of timers that can use sim time when possible, and not be strictly held to wall time. This is already possible with rclcpp::create_timer and passing in the right arguments, but having it as a function of rclcpp::Node will make it more convenient to use.

Implementation considerations

Add a Node::create_timer declaration into the node.hpp file, but since it will take a callback the definition will most likely need to be added to the node_impl.hpp template file.

The implementation will most likely just be a wrapper around one of the rclcpp::create_timer calls and passing the correct arguments as needed. I don't know which of the create_timer functions will be better to use.