Rework DSPy retrieve

[chenmoneygithub]

Right now dspy.Retrieve is not a useful abstraction, and we have too many prebuilt dspy.Retrieve implementations. Although it would be nice to have DSPy play as a centralized hub for these retriever providers, this will quickly go outdated due to the maintenance challenge. Before our code smells, we are reworking the dspy.Retrieve with the goal that:

• dspy.Retrieve becomes a useful abstraction, which can help people instrument retrieval (with callback support), and save the retriever imlplementations.
• Users find it easy to provide a custom function to do retrieval, without needing to worry about converting their code to DSPy's rules. It will be really bad if we force people to learn a over-engineered dspy.Retrieve.
• The interface is possible for future retrieval optimization, and in general expansible.

Meanwhile, we realized that many retrievers require users to compute the embedding before querying, so we are exposing a dspy.Embedding interface, which is built on top of litellm, similar to dspy.LM.