Add Support for Writing into tokio `Bytes`

[InsertCreativityHere]

Currently our OutputTarget trait is only implemented for &mut [u8] and &mut Vec<u8>.

It would be good to add a specialized implementation for the Bytes type provided by tokio's bytes crate.
This type appears in Quinn's stream APIs and also sees use in prost and tokio projects.

So, even if we might not be using it ourselves, supporting it would enable more people to try our crate with less friction.