gptel: Fix prompt string handling in gptel-request

* gptel.el (gptel-request): When `gptel-request` is supplied a
string, it creates the full prompt plist according to the OpenAI
API.  Fix by inserting it into a temp buffer and using the
cl-generic dispatch to parse the buffer instead.  This is a janky
solution but the best possible one without defining another
generic function just to handle prompt strings differently per API.

gptel.el

@@ -637,8 +637,12 @@ Model parameters can be let-bound around calls to this function."
             (let ((gptel--system-message system))
               (gptel--create-prompt start-marker)))
            ((stringp prompt)
-           `((:role "system" :content ,system)
-             (:role "user"   :content ,prompt)))
+            ;; FIXME Dear reader, welcome to Jank City:
+            (with-temp-buffer
+              (let ((gptel--system-message system)
+                    (gptel-backend (buffer-local-value 'gptel-backend buffer)))
+                (insert prompt)
+                (gptel--create-prompt))))
            ((consp prompt) prompt)))
          (info (list :prompt full-prompt
                      :buffer buffer