Programmable timer for boiler without touching thermostat
You need a relay, which is a usually part of a timer - but you need a timer which has separate contacts for its power, and the relay contacts, rather than a plug-in type timer that combines those.

This is not an uncommon configuration, but shopping questions are off-topic so you'll have to find one yourself. I wlll make a general comment that "DIN" relays (designed for mounting on a "DIN Rail" as part of industrial control solutions) often appear to be the most reliable form of these, but there are other form factors available.

You could also use a "plug-in timer" connected to a separate relay, but that's more parts than you actually need.

Concern: If you turn off the system completely while you are away, what happens if the temperature drops below freezing? Could be rather messy, which is why most systems are simply set back to 50-55F or 10-12C rather than being turned off, at least in heating season.