Support ExtendedPostiion and Current control; Implement on_configure

[sebtiburzio]

Hello, last year I worked on a project using dynamixel_hardware, during which I came across some problems and made some changes:

• Replaced the use of Position mode with ExtendedPosition mode
• Added support for Current control mode
• Included velocity and effort state updates in dummy mode
• Implemented the on_configure callback for initial communication with motors (instead of on_init, to align with this diagram: https://control.ros.org/rolling/_images/hardware_interface_lifecycle.png)

Couple of things to note:

• The hardware interface implementation doesn't handle the different control modes very elegantly and will switch between them when receiving different command types. So I would recommend ensuring only one controller is active on startup and providing ROS services to switch between them.
• According to the ROS2_control diagram linked above, powering the motors (enable_torque(true)) should be done in the on_activate callback. However I found that after on_configure completes the real-time read/write calls interfere with the dynamixel workbench functions, so I left it in on_configure. I believe similar issues are discussed here: Delayed Read Write Calls till after on_activate() is completed #89

Feel free to review the changes and see if they can be included. I am no longer working on the project (and don't have access to the hardware) but can make some updates if needed. I also noted there has been some other PRs made recently, not sure what the compatibility is with my changes.

Cheers.