Cosmos: Review SDK Response types

[analogrelay]

I want to do one more pass over the SDK response types (ItemResponse, ResourceResponse, etc.) before locking those APIs. The main thing I want to consider is creating a type representing everything except the body for a given response. Because the body is stored as bytes but users want it as their model type T, we consume the response in into_model() (to avoid copying the bytes). It would be good if there was a way to "split" the response into "headers" and "body" and then only consume the body to deserialize.