ChatGPT constructor: add slow-down option so that it can also be used if you don't have a paid ChatGPT account

##### Prototypes version
0.21.1

##### Orange version
3.36.2

##### Expected behavior and actual behavior
I wanted to try the [cookie-mining example](https://orangedatamining.com/blog/cookie-mining-2023/), but, as a non-paying ChatGPT 3.5 user, I'm not allowed to send more than 3 requests per minute. It would be nice to be able to try this out, even if it means that the workflow has to run overnight. So perhaps add a checkbox "reduce requests per minute (for non-paying users, slow!)" ...

Start by locating the ChatGPT constructor used by the cookie-mining example and trace where requests are issued. Add a user-selectable slow mode for non-paying accounts, then verify that requests are limited to about three per minute while the workflow still completes.

Written by the indexing model from the issue text.