Add SSH key authentication with keypath

[DeniedPyro]

I saw that the library lacked a high level wrapper for key authentication, I've only added one that uses the filepath of the key!
Feel free to add changes/suggestions!

Use case:

import LibSSH as ssh

session = ssh.Session("foo.com")
key_file = expanduser("~/.ssh/id_ed25519")
ssh.userauth_publickey_file(session, key_file)
close(session)

[DeniedPyro]

I can't really run automatic tests on this, since DemoServer only accepts password authentication.

I also can't assign a reviewer on this, @JamesWrigley if you have time to look through it, it would be really appreciated:)

[JamesWrigley]

Hmm looks like there are some other issues in CI. Probably unrelated to your change though. I'll have a go at fixing CI in the next couple of weeks (but feel free to bump this if I forget 😛).

[JamesWrigley]

Quick update, #33 is close to being ready so I'll try to finish that first, and then rebasing this PR should get CI in the green again.