Demo: custom function that uses GPT API

[sequba]

Add a working example that demonstrates creating a custom function that calls an external API by implementing GPT() function - a GPT API wrapper.

Docs: https://platform.openai.com/docs/introduction
Pricing: https://openai.com/pricing

Internal discussion about it: https://handsoncode.slack.com/archives/C031GBX62SE/p1710595153475939