feat: gpg option arg completion improvements

[scop]

Contains a somewhat unrelated style change which I'm too lazy to submit separately :P

[akinomyoga]

I'm not familiar with these GPG keys but it seems there are more options that receive keys as arguments.

• --list-keys / -k / --list-public-keys, --list-secret-keys / -K ... optionally receive keys as arguments?
• --locate-external-keys ... synonym to --locate-keys
• --delete-secret-keys
• --search-keys
• --export-secret-keys, --export-secret-subkeys, --export-ssh-key ... some variants of --export, but it is not explicit whether they accept keys in arguements.
• --try-secret-key

[scop]

Thanks! Added all but --export-ssh-key, as I didn't have any keys around it would work with.