Operation request: Add PGP Sign

## Summary
The PGP operations in CyberChef currently have "Encrypt" and "Encrypt & Sign". There's no only Sign operation.
Please add a PGP Sign operation which only needs the PGP Private key and Private key passphrase.

This allows to only sign and not encrypt a file.

Start by locating the existing PGP Encrypt and Encrypt & Sign operations, then trace how they accept the PGP private key and passphrase. Add a PGP Sign operation that signs without encrypting and verify that it works with only those credentials.

Written by the indexing model from the issue text.