Have `CreateRequest()` return a `*Request`

At the moment this function returns the serialized request and, separately, the nonce used to verify the response. One way to clean up this API is to return a `*Request` that the caller serializes itself before sending to the server.

_Originally posted by @lukevalenta in https://github.com/cloudflare/roughtime/pull/59#discussion_r1708239188_