Add support for the providers OpenAI and OpenRouter

Is your feature request related to a problem? Please describe. Provider support is limited to Groq.

Describe the solution you'd like Add support for OpenAI and OpenRouter by:

Switching to the OpenAI library, which is compatible with Groq, OpenAI, and OpenRouter.
Allowing users to choose their provider by specifying the API base URL using the options --base-url or -u, or through an environment variable.

If no provider is specified, a default should be set.

Describe alternatives you've considered N/A

Additional context N/A

uday-rana / codeshift