GH Issue: https://github.com/mongodb/bson-rust/issues/304

The BSON library currently supports synchronously reading Document from a reader of BSON bytes via Document::from_reader, but it provides no such support for doing so via async. We should investigate adding support via something like Document::from_stream or something.

One challenge is that there isn't an async Read equivalent in the standard library. There are a few crates that include such a trait (e.g. futures), but it's unclear if it meets our stability requirements.