[FEATURE] Allow using PGP key for SSH

[skewballfox]

Is your feature request related to a problem? Please describe.

With OpenKeychain it was possible to use your GPG key to authenticate over SSH, which is not supported with the PGPainless backend.

Describe the solution you'd like

Implement support for using a selected GPG key as the SSH authentication method.

Describe alternatives you've considered

N/A

Additional context

N/A

[vanitasvitae]

I can look into adding support for this to the library.