[Go] Support for OpenAI image URL

[emranbm]

Is your feature request related to a problem? Please describe.
Using OpenAI plugin in Golang, ai.Part with the Kind of ai.PartMedia does not seem to support ImageURL as the content.

Describe the solution you'd like
It'll be great to support image URL (e.g. https://google.com/image.png) without requiring it to be downloaded / encoded. The issue can be addressed through leveraging the ai.Part.Custom field.

Describe alternatives you've considered
Of course, one way is to download and encode the image, although it forces a heavy bandwidth on the clients.

Thanks for the great kit!