Create a new workflow

Optional client-generated key, 1-255 visible ASCII characters (a UUID works). The first request the server accepts under a key creates one workflow; a retry with the same key and an equivalent body gets that request's status and body back (with Idempotent-Replayed: true) and changes nothing. The key is scoped to the caller's identity and this operation; it is not a credential. The same key with a different body is a 409. A request rejected before it is accepted (validation, auth, not found, disabled) does not use up the key. Accepted keys do not expire.