Add SSH key auth for SFTP exports

You can now authenticate SFTP dataset exports with an SSH private key, in addition to the existing username and password option. When you create an SFTP export credential, choose your authentication method: username/password, or SSH private key.

To use key-based auth, install your public key on your SFTP server, then enter a username and paste your private key in NinjaCat. Only unencrypted (passphrase-free) private keys are supported. Existing username/password SFTP credentials continue to work unchanged, and this does not affect Amazon S3, Google Cloud, or Azure exports.