Add LED support

[lukicdarkoo]

There are 4 RGB LEDs (LED2, LED4, LED6, LED8) and 4 on/off LEDs (LED1, LED3, LED5 and LED7). These LEDs should be exposed as subscriptions.

I was thinking to use services instead of topics as it is done in webots_ros2_epuck2 but:

• There is no standard, suitable, service for this usage. One should install our custom service message just to control it.
• There is no need for the return value, LED cannot fail to turn off/on.

Therefore, webots_ros2_epuck2 should be updated accordingly as well.