[Feature] Host key validation

[Feature] Host key validation

## Expected Behaviour

Most solutions which consume SSH will always, strictly validate SSH Host keys, to make sure that the host being connected to by the client is as expected.

I suspect that most of the time, people type in "yes" without looking at the host key, and without verifying it.

After accepting the Host key, it gets stored in `~/.ssh/known_hosts` on its own line. Whenever the user reconnects, that known host value will be used to validate that the Host key has not changed. If it has changed, then the ssh client will refuse to connect, since the host may have been tampered with, or the connection may have been intercepted.

## Current Behaviour

k3sup ignores hosts keys, since it's designed to be used headless, without any prompting. Forcing strict acceptance of host keys will require user interaction and break the user-experience.

## Possible Solution

As a middle-ground, k3sup could accept the host key, and maintain its own known_hosts file, or make use of the the system-level file.

A downside to this is that k3sup is often used with VMs, for learning, with RPis, and with labs, where the IP and host key will rarely match between requests. k3sup also is designed to connect to each agent host only once, which defeats the purpose of the host key validation.

This is not a simple issue, however I hope that security-types will be reassured that the potential issue is being logged, and that suggestions are welcome.