feat: allow users to give custom base urls

[hsethi2709]

I typically use GPT using a different base API URL - I am trying to add a change here so the extension can support accepting a different base URL but will use the original one by default.

Genie, VS Code extensions, also allows you to give a different base url since a lot of companies use a proxy in front of openAI.

I saw that chatgpt npm package supports providing a different custom URL so I have used that here for this purpose.

I am new to the extensions world so if you think something can be done better or this is not going to work, please feel free to leave a comment and I'll try to fix it.