CreateRequest creates a Roughtime request given an entropy source and the contents of a previous reply for chaining. If this request is the first of a chain, prevReply can be empty. It returns the nonce (needed to verify the reply), the blind (needed to prove correct chaining to an external party) a
(versionPreference []Version, rand io.Reader, prevReply []byte, rootPublicKey ed25519.PublicKey)
source not stored for this graph (policy: none)