feature: Add health check endpoint

[matthiasschoger]

First of all, great project! This enhancement request should be pretty easy to implement.

For added resiliency, add a health check endpoint to validate that the updater is actually running.
Proposed endpoints could be /health or /alive, with a simple HTTP 200 as the response code. Maybe add an "ok" in the body.

Bonus points: A JSON response which lists if the last communication attempts to the Fritz!Box and Cloudflare were successful. Still keep the 200 response code, even if these connections were not successful.

[cromefire]

Would definitely take a PR, not sure when I have time to implement that, pretty busy right now... Ideally it'd also expose some Prometheus metrics with failures and such I think so you can set up alerting