Minimize required Headscale configuration schema

[tale]

Description

Currently in code we keep track of the entire Headscale configuration schema for "safety purposes". This might actually be such a silly idea I'm not sure what I was thinking when I originally made it. Instead, only the keys/values we actually read/write to need to be there and NEED to support version enhancement similar to how our API runtime client can in order to maintain our version matrix compatibility.

Importantly, that needs to be unit/integration tested no matter what.