Changing how handles are used (in stages and PRs)

Add a new method for exporting interfaces that enables exporting only names and types without pointers to storage (doubles still in the background).
- Add getters and setters for those values. Keep backward compatibility to people that export handles with pointers. #1240
- Decide where to store values and how to provide them to read and write:
  1. Add them to the HardwareInterface interfaces?
  2. Add them to Hardware Components, i.e., System, Sensor, Actuator, and then as arguments to the implementations
- Deprecate the old export.
- Add migration guide in control.ros.org. (Partially https://github.com/ros-controls/ros2_control_demos/pull/396)
Decide how to access those new handled in the read and write methods
1. Add arguments to read and write that provide map or list of interfaces that people access to
2. Add implicit access (confusing for users)
Enable vector and type handling by providing information from users and URDF tag. In the background, we will use doubles (and maybe chars if necessary because of double interpretation errors).
Move everything to new handles that support variants, i.e., optional.
Remove the support for the old functionality

ros-controls / ros2_control