fixes #550

The biggest change (this will break existing API) here is that a rcl_lifecycle_transition_key_t now is a struct has thus has label and id. This label will then be presented when calling get_available_transitions. There is a second service introduced called get_transition_graph which yields all available transitions within the state machine. This is then helpful when introspecting the state machine.

I am not feeling a hundred percent convinced that the naming here is perfect, so I'd ask for feedback between the following separation:

• Transition: A unique transition within the state machine is labeled as i.e. configure_to_shutdown or configure_to_configuring.
• Key: A transition key is used here to trigger the change of state, i.e. configure or shutdown and one key can be used to trigger different individual transitions. The example here could be shutdown which can be used to trigger configure_to_shutdown or inactive_to_shutdown.

Any feedback on naming here?