Add the private key's passphrase as a field to the SSH key item

@smanojkarthick Thanks for your feedback. Currently, importing a private key decrypts it and stores it in 1Password as an unencrypted key. The key is still encrypted by 1Password, but if you were to export it, it would export as an unencrypted OpenSSH key instead of the original format.

Given that the passphrase is no longer needed for the key, would you still like to store it on the key? Thanks!