recv_data should return concrete buf impl not generic

[taikulawo]

similar like #93 but for recv_data.

Current h3 return Bytes but poll_data signature use impl Buf, it's hard for outside return owned concrete type without pay copy using Buf#bytes().

d is Bytes.

h3/h3/src/frame.rs

Line 116 in dbf2523

self.remaining_data -= d.remaining();

h2 crates return Bytes, not impl Buf, it's good.

pub async fn data(&mut self) -> Option<Result<Bytes, crate::Error>> {
      crate::poll_fn(move |cx| self.poll_data(cx)).await
  }

send_data should support generic over T, but should we also use generic on recv_data?

[taikulawo]

Well, it's seems Bytes use shadow copy buf.map(|mut f| f.copy_to_bytes(f.remaining()))