Add simple healthz handler

[bakins]

For #93

This creates a very simple HTTP endpoint to be used for liveness and readiness checks. In this initial version, it only returns OK. In the future, we may want to check additional functionality before returning OK.

We can reuse this listener for things such as prometheus metrics, as mentioned in #23, in the future, if desired.

[linki]

That looks good.

I was wondering – since you already mentioned /metrics – what's your opinion about changing the flag name to --metrics-address similar to ExternalDNS? Just --address seems a bit generic and "is it healthy: yes or no" can also be considered a metric, imo.

Let me know what you think.

[bakins]

@linki --metrics-address sounds good to me. I'll make that change. Also, in another PR, I can add basic Prometheus metrics if that would be useful.

[linki]

@bakins yes, please do that.

[linki]

Thanks @bakins 😄