Added ability to override the OpenAI API endpoint

[wkok]

Discussion refers: https://clojurians.slack.com/archives/C054XC5JVDZ/p1693981662110889

Sometimes it might be needed to override the :api-endpoint for the OpenAI API

This PR allows that by either specifying it in the options map for each api call:

(create-completion {:model "text-davinci-003"
                    :prompt "Say this is a test"}
                   {:api-endpoint "https://my-new-endpoint.com"})

or once off as an environment variable:

export OPENAI_API_ENDPOINT="https://my-new-endpoint.com"