send a heartbeat every 60 seconds to keep serial connections alive

[liamcottle]

This PR sends a heartbeat packet every 60 seconds to keep the serial connection alive.

Without this, after a while (15 minutes or more), packets stop being received when connecting a Pi Pico to a Meshtastic device over serial.

[ianmcorvidae]

Unsure if it's worth harmonizing between clients, but python and rust both use 300 seconds for the heartbeat interval. It does look like js is using 60s, but wondering if that's too fast really.

[liamcottle]

I hadn't looked at the implementations of those clients for heartbeat, so just assumed the value in the js lib was what I should go with. I'm happy to push another commit to adjust this to 300 seconds if preferred.

This would only allow for 3 failed serial packets going through, as opposed to 15 ;) but unsure if that matters...

[GUVWAF]

60 seconds sounds good to me.