Support custom endpoint for OpenAI

[katopz]

🚀 The feature

OpenAI not consume endpoint
https://github.com/embedchain/embedchain/blob/3cff5e9/embedchain/llm/openai.py#L28-L33

        kwargs = {
            "model": config.model or "gpt-3.5-turbo",
            "temperature": config.temperature,
            "max_tokens": config.max_tokens,
            "model_kwargs": {},
        }

So that i can't customize it to other endpoint.

Motivation, pitch

Support custom endpoint config.