Added Server.KeyboardInteractiveHandler

[aidansteele]

I needed this in order to implement functionality similar to https://github.com/FiloSottile/whosthere#how-it-works (i.e. I wanted to print a custom error message for users who failed to present any acceptable public keys), so I added this handler.

I'm submitting it for your consideration because having it upstream means less work for me, but understand that it's pretty niche and might not be in the spirit of this project. Feel free to use or not :)

[belak]

This seems reasonable to me. It is a little bit of a niche, but it's fairly simple to add and exposes a feature of the official package without complicating anything.

[belak]

I'm fine with merging this in for now, but note that it may be changed at a later date since we're looking at revamping how config works. Thanks for your contribution!

[aidansteele]

That’s fair enough, and thank you! 💯