Add Iterators to Custom Objects

[jeffistyping]

The Python response objects that just wrap around a list object (BestChoices, Embeddings, Similarities) should be iterable.

For example, Embeddings returned from the Embed endpoint can be iterated through by using:

# embeddings = co.embed(model="baseline-shrimp", texts=["co:here", "cohere"])

for em in embeddings.embeddings:
      print(em)

Users should expect to be able to do this instead:

for em in embeddings:
      print(em)

[jimwu6]

Added comments to separate stuff in sanity-test.py, so take a look

LGTM otherwise