Support certificate based authentication

[jeanp413]

See #87 (comment)

[whazor]

Seems to be blocked by: mscdex/ssh2#551

Certificate authentication works fine for me with using pageant and wsl-ssh-pageant in cobination.

https://github.com/benpye/wsl-ssh-pageant
https://www.putty.org/

[murkvin]

I posted a Pull Request which could mitigate this issue for some folks.

jeanp413/ssh2#1