[Cosmos] Integration tests for items and query

[analogrelay]

This PR adds integration tests for Item CRUD and Item Query to the Cosmos crate, somewhat inspired by the emulator_cosmos_item_test and emulator_cosmos_query_test.

I did want to break down some of the operations to their own tests though, unlike the Go SDK which has a bundle of "CRUD" tests in one test. We can always pivot to that if necessary for test performance, etc.

In addition, I changed our doc examples to use ? syntax (by having them be in async fn doc() -> Result<(), Box<dyn std::error::Error>> wrapper functions). It looks a lot cleaner that way, IMO, by removing all the noisy .unwrap() calls and matching more closely to what we expect users to do in their own applications.

Finally, I did need one change in the azure_core_test crate, because our live tests were broken by #1956 . I changed azure_core_test::recorded::start to return Result<Option<Session>> (instead of just Result<Session>). If the test is running in live mode, it'll return Ok(None), to indicate that no test proxy session was established. I think this is the right way to handle this, but I'm open to feedback.

I'm not expecting action on this until the new year, because I know @heaths is out until the new year and the azure_core_test change will need his ✅ , but I wanted to open it up and get Cosmos folks looking at it.