Change interface export variant

[mamueluth]

This showcases for example 1 how the changes proposed in this PR would affect the hardware. The adjusted example has been tested and works with the changes. If you want to test them, first checkout the branch from the before mentioned PR.

The changes are described in more detail in the PR:

• Storage for Command-/StateInterface is moved to the SystemInterface -> no local vectors of commads/states in the hardware (memory for Command-/StateInterfaces)
• The exportation of the Command-/StateInterfaces has been removed and is handled by the Framework -> removed export_state_interfaces() and export_command_interfaces() in the HW
• SystemInterface provides set_state/get_state and set_commad/get_command methods to update states/read commands in the HW. Can either be used by passing the InterfaceDescription or state_interface-/command_interface-name

If you want to test:

1. Checkout the change_interface_export_variant branch from this PR
2. Follow the instructions here to run the demo.

ros2 launch ros2_control_demo_example_1 rrbot.launch.py
ros2 launch ros2_control_demo_example_1 test_forward_position_controller.launch.py