pzem-004t v3 (pzemac) timeout option

When the PZEM-004T V3 loses AC power it stops responding to esphome due to it requiring AC power.
It would be handy to have a timeout option that either zeros out all the sensor values or sets them to unavailable if the sensor does not respond.

This would be very useful to detect AC power failures without adding additional components. Especially when the ESP has its own power source.

Start with issue #1694 and trace the ESPHome PZEM-004T V3 component, focusing on how sensor responses and timeouts are handled. Determine the intended behavior when AC power is lost, then verify that nonresponsive sensors consistently become zero or unavailable and that AC power failure can be detected.

Written by the indexing model from the issue text.