[Feature] Use GPG --export-ssh-key to use with git authentication

[parrot7483]

Since GPG 2.1.11 it is possible to export a SSH key from a GPG sub-key with authentication usage. This exports a SSH public key which than can be used to authenticate with git. Using this function a user does not have to import a SSH key. Also some users might not even use native SSH keys anymore because of the convenience of just having to manage one type on key.

Possible Problem: The key has multiple sub-keys with authentication usage and not the first one is used to generate the SSH key. GPG does use the first by default. App would need a way to select the right one.

More information: What’s new in GnuPG 2.1 Copy from gpg man page for --export-ssh-key

          This command is used to export a key in the OpenSSH public key format.  It requires  the
          specification  of  one  key by the usual means and exports the latest valid subkey which
          has an authentication capability to STDOUT or to the file given  with  option  --output.
          That output can directly be added to ssh's ‘authorized_key’ file.

          By  specifying the key to export using a key ID or a fingerprint suffixed with an excla‐
          mation mark (!), a specific subkey or the primary key can be exported.   This  does  not
          even require that the key has the authentication capability flag set.