Allow supplying/overriding base URL of openai provider

Currently, BricksLLM seems to have hardcoded the OpenAI base URL everywhere in the code as https://api.openai.com.

But with OpenAI's Data Residency feature, projects in the Europe region need to use https://eu.api.openai.com.

It should be possible to create a new custom `openai` provider with a different base URL, or at least change the base URL for the default `openai` provider.