tokio_quiche: Unable to use a custom `BufFactory`

Hey folks, I'm building a async friendly API using [Bytes](https://docs.rs/bytes/latest/bytes/struct.Bytes.html). I implemented `quiche::BufFactory` for it via a wrapper. A `bytes` feature would be nice to avoid this wrapper, but that's a separate request.

However, I don't think it's possible to use because [ApplicationOverQuic](https://docs.rs/tokio-quiche/latest/tokio_quiche/trait.ApplicationOverQuic.html) is not generic over `F: BufFactory`. The `quinn::Connection` passed to the trait (aliased as `tokio_quiche::QuinnConnection`) uses the default `DefaultBufFactory`. Or `tokio_quiche::buf_factory::BufFactory` with the `zero-copy` feature.

I'd like to use [stream_send_zc](https://docs.rs/quiche/latest/quiche/struct.Connection.html#method.stream_send_zc) with `Bytes` to avoid the copy. I imagine the `zero-copy` feature exists for a reason and there's probably some weird Rust quirk preventing generic zero-copy support, but I just wanted to ask anyway.