Refine reverse proxy for kea

When we implemented Kea Control Agent we considered whether we should provide a built-in solution for https or not. At that time, it seemed reasonable to rely on third party HTTP servers, such as nginx to provide reverse proxy function because we don't have the maintain the security critical code. The third party HTTP servers are trustworthy when it comes to security. However, we've had reports that installing such a third party software is not always possible, especially when talking about large installations.

This ticket resumes the discussion how we could possibly provide a viable alternative to third party reverse proxies and ship something with Kea that would provide similar function and would be easy to install, deploy and run.

A discussion/high level design page is here: https://gitlab.isc.org/isc-projects/kea/-/wikis/designs/https-wrapper-for-control-agent-discussion