Removing async_trait following stabilization of async traits fn in Rust 1.75

[Samuel-B-D]

With the stabilization of async traits fn ( https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html ) it should be possible to remove the boxing of the futures in the traits. trait_variant could be used to provide non-Send/Sync versions if the mongodb sync api should still be supported.

I suspect/hope that this might bring some small performances improvement by not boxing the futures (?)