Add typed signatures

[michaelisaac-dev]

Adds a typed Signature[TInput, TOutput] API that lets users define modules with input/output contracts using dataclasses or Pydantic models.

    @dataclass
    class InputType:
        question: str = dspy.Field(desc="The question to answer")

    class OutputType:
        answer: str
        confidence: int = dspy.Field(desc="How confident are you in your answer?")

    signature = dspy.Signature(input_type=InputType, output_type=OutputType)
    program = Predict(signature)

    response = program(InputType(question="What is the capital of France?"))

    assert isinstance(response, OutputType)  
    assert isinstance(response.confidence, int)
    assert isinstance(response.answer, str)