ipc: add function to retrieve public keys from gpg-agent

This Merge Request will add a function to retrieve the public keys from the gpg-agent.

This is useful when you don't have the public key (like the key is on a smartcard or it's from the user input) but you know the Keygrip.

Currently, it's a WIP, since I don't know how to differentiate EDDSA/ECDSA/ECDH public key type from the agent response alone.

Update: see the new comment below.