User request: https://github.com/mongodb/mongo-rust-driver/issues/1098

I propose we make this behavior opt-in by adding a with_type method to the Aggregate action. I believe this is the first request we've had for this behavior in the ~3 years since introducing serde integration, and more generally the flexibility of the data types returned from an agg pipeline makes Document a reasonable default. Adding an outright generic would require a type label for all calls to aggregate.