Cache passphrase-encrypted SSH identities

[veehaitch]

rage recently introduced support for passphrase-encrypted age identities. Naturally, the implementation prompts the user for entering the passphrase of the age identity. As far as I understand, rage caches the identity once decrypted successfully using the entered passphrase. This comes in handy when using the rage crate to decrypt multiple inputs as consecutive calls will use the cached identity without prompting the user for the passphrase.

Apparently, however, rage treads passphrase-protected SSH identities differently asking for the passphrase on each use of the key. It would be helpful (and also consistent) to cache SSH identities as well.