This is behind a feature flag, since it isn't needed for pure clients: only onion services and relays need this.
I've named the object that constructs these certs
Ed25519CertConstructor because it doesn't follow the builder
pattern exactly: mainly because you can't get an Ed25519Cert out of
it. That part is necessary because we require that an Ed25519Cert
should only exist if the certificate was found to be well-signed
with the right public key.
Closes #511 (closed).