EVSE: Add external current watchdog?

[rtrbt]

The charge managemer implements a watchdog for the available current API. EVSECommon could implement a similar watchdog for the external current API, allowing a user to either block or release the slot if it was not written for a long time.

• Do we want this at all?
• Do we want this for other charging slots? (Modbus TCP, etc)
• Maybe refactor the charge manager watchdog to watch any slot.
• Maybe implement this as a cron trigger? -> Less visible configuration to confuse users, more options to handle a watchdog timeout.

[rtrbt]

Implemented as cron triggers.