[WIP] Add gpio support in gz system

[Nibanovic]

Implemented gpio state_interfaces in gz-system.

In the current implementation, it is not necessary to specify a separate state and command interfaces in the gpio, but this exports state_interfaces as commands as well, under the same names.
This declaration in urdf...

<gpio name="proximity_sensor">
        <state_interface name="state">
          <param name="initial_value">0.0</param>
        </state_interface>
        <state_interface name="state2">
          <param name="initial_value">0.0</param>
        </state_interface>
        <state_interface name="state3"/>
</gpio>

... results in this output:

Loading GPIO: proximity_sensor
        State:
                 state
                         found initial value: 0.000000
                 state2
                         found initial value: 0.000000
                 state3
        Command: (mocked)
                 state
                 state2
                 state3

Testing

• verified instantiating multiple gpios
• verified that the state mirroring works with our proximity_sensor_controller, which claims the state_interface as expected, and reads the proper value after I alter it with forward_command_controller