feat: adds Unify AI

[OscarArroyoVega]

This feature adds support for using Unify AI Using Unify gives support to a wide variety of AI Models with 1 single unified API key Unify enables routing and training of custom routers across various models and providers to optimize performance for specific prompts.

model = dspy.UnifyI(model="gpt-4o@openai", 
          api_key=<your_key>
)
dspy.settings.configure(lm=model)
# ...use normally...