Do argument parsing outside of node constructor

Feature request

Things that come from command line arguments should be separately passed into the node's constructor rather than passing in arguments and asking the node to do the parsing.

Feature description

Currently command line arguments may be passed to a node via an argument to the node's constructor. This separates the responsibility of parsing arguments from actually using them. An entity like ros2 launch launch composable nodes would invoke parsing of any command line arguments, and then pass the results into the node.

Implementation considerations

The current arguments passed to the node are https://github.com/ros2/rclcpp/blob/8f793fdb4aa6d21ea2442a31547546a3c90692a5/rclcpp/include/rclcpp/node.hpp#L94-L101

The information that should be passed in if not through the arguments is:

• Topic and service remap rules
• node name and namespace remap rules __ns:=..., __node:=...
• Initial parameter values __params:=...
• Log level (ros2/ros2#498)

Since new features may be added as command line arguments it may be an advantage to pass rclcpp::NodeArguments() so future features don't have to change all the places where arguments are passed to a node's constructor

requires ros2/rcl#254