Generates a DKIM key and prints the TXT record to publish.
mix mail_auth.gen_key --domain example.com --selector s1
mix mail_auth.gen_key --domain example.com --selector s1 --ed25519 --out s1.pemOptions:
--domain,--selector: where the record goes--ed25519: an Ed25519 key instead of RSA--bits: the RSA key size. Default 2048--out: write the private key to this file instead of printing it