Support setting log levels in agentk via env vars
Relates to #496 (closed).
I often find myself editing the constants with defaults because there is no way to set the log levels early, before the config is fetched. This is not even possible when kas connects to agent or when connectivity from the agent is not working. So, it makes sense to make it possible to set levels via env vars.
Note that there is no such problem with kas as it just reads the config file on startup.