add heartbeat thread

[miDeb]

• send heartbeat requests at regular intervals to all nodes
• register heartbeat responses

Things to be done/defined:

• How should we handle unresponsive nodes? Should we try to detect that, or do we just note down received heartbeats as I did here?
• Should we check if the received heartbeat value matches what we expected? What do we do if it doesn't?

[miDeb]

We could also save a bit on bus bandwidth by not sending a heartbeat_req to each node individually but just broadcast it to all nodes (of course that would mean we can't have a heartbeat counter for each node individually)

[raffael0]

Nice work! Thanks ❤️

I think for now it's fine to just note down the time when we last received a heartbeat. Later on we should periodically check that and send the node state to the ECUI.

Should we check if the received heartbeat value matches what we expected? What do we do if it doesn't?

Good question. I don't think we should do anything actively if it doesn't match. Once we pass the status messages to the ECUI, we can also define some Ferroflow messages and this should be one of them.

Broadcasting messages

I don't think this will be the biggest bandwidth eater so i'd just leave it as is for now