Make known_hosts optional about ssh-key-action

[shimataro]

You can use known_hosts: unnecessary.

known_hosts is not optional.
It's designed intentionally because It should not be omitted casually (as you know, it is insecure to connect except github.com without known_hosts).

If you connect only to github.com, use known_hosts: unnecessary explicitly.

closing.

[nicolas-albert]

You should include this case in your main page : https://github.com/marketplace/actions/install-ssh-key

I found known_hosts: unnecessary just here with luck !
Also put this as a default value isn't a bad idea !

Thanks for your plugin.