feat(py/genkit): added the resolve_method for openai compatible plugin

[hendrixmar]

Description:

This PR introduces the concrete implementation of the resolve_method for the OpenAI compat plugin.

[AbeJLazaro]

Let's check if it is required to keep using the handler. I think the model generate method is enough, and the handler is only a wrapper to determine which models are valid based on the supported models variable. We are going to deprecate those validations, so maybe the handler is not required.