Converts OpenPGP Curve25519/Ed25519/Ed25519Legacy secret to ssh secret
For RSA use monkeysphere (openpgp2ssh)/openssl
Use gpg-agent/ssh-add to convert in opposite direction.
Example ssh CA issuing of user certificates from PGP web of trust
ssh-keygen -s /dev/stdin -I [email protected] id_file.pub \
<<< "$(gpg --export-secret-key <pri key id> | \
sparkhill <sub key id>)"