Add support for PGP authorized keys usage

SSH 2.3 seems to have added the capability to use PGP keys as authorized ones:

SSH2 Version 2.0.13 introduced support for PGP authentication. Your authorization file may also include PgpPublicKeyFile, PgpKeyName, PgpKey Fingerprint, and PgpKeyId lines. A Command line may follow them, just as it may follow Key:

# SSH2 only
PgpKeyName my-key
Command "/bin/echo PGP authentication was detected" 

Some examples of how to use Bouncycastle to facilitate this:

• Sample code
• jpgpj Library wrapper

Python converter pgp->ssh

openssh-gpg configuration

Calculating PGP key fingerprint