Implement ICMP-based host checking

[krustowski]

The proposed usage of ICMP would mean utilizing a "more native" way of how to check a generic host's availability/reachability. At the moment, we tend to use an SSH port check to do the same thing.

[Tackx]

TBC: either use ICMP when http is not specified and no port is specified either in the JSON or add a new field (e.g. type). The first option would be backwards compatible.

[krixlion]

If no one is currently working on this I'm willing to implement this feature.

Since the standard library does not currently have utilities for parsing ICMP messages I'd have to consider importing either the official experimental ICMP module or some other third-party module. Is the zero-dependency requirement on dish non-negotiable?

I can write it from scratch but by doing so I'd reinvent the wheel and add a considerable amount of complexity to something which otherwise could be a rather simple feature. That being said it sounds like a fun exercise.

[Tackx]

I think we would prefer to keep the zero deps label, but ultimately it should be decided by @krustowski.

[krustowski]

Hm, my bad. I haven't considered the complexity behind ICMP implementation in Go atm well. I tried using that experimental module once, which is quite useful. But yep, it would require using it as a dependency. And I would rather go on with the zero-dep approach.

Thank you for your involvement, @krixlion