feat(completion): support for SSH hosts

[Sporif]

Retrieve the hosts from the hosts file, SSH config files, and SSH known_hosts files. Also add a setting to allow users to disable hosts file completion in case it contains too many hosts.

The implementation copies _comp_compgen_known_hosts__impl from bash-completion as much as possible. But some parts are intentionally left out, e.g. ssh2 config paths, avahi-browse, ruptime. I believe it also improves upon the ssh config parsing since the lists of hosts from nixos-cli is more correct, at least on my system.

I would use the ssh_config pkg but I'm not sure how to recursively get all hosts, due to some unexported fields. It's probably why a PR was made to add a GetHosts function. So instead of using a fork I have implemented partial ssh config parsing, which has been very fun.

Closes #186.

[water-sucks]

While I do appreciate the time taken for this, it does seem like an awful lot of code to have to introduce that we would have to maintain ourselves, just for SSH config parsing.

I'm open to including it for now, but we may want to pivot to a more battle-tested approach later on because I am sure we may run into issues in the future when users with more complex SSH configurations start to use it.

Also, FYI there is already precedent to use forked packages here, which I use for go-systemd for adding some extra systemd dbus methods, so i don't particularly mind if we introduce a new dependency for this completion if things aren't getting merged upstream, especially since the ssh_config dependency you mention itself has zero transitive ones. Let me know what your thoughts are on this and I'll review the code afterwards.

[Sporif]

Ok, you convinced me to create a fork for ssh_config and use the hostsfile pkg for the hosts file. I also added user@hosts completion.