feat: add mutable buffer api to FramedRead

[driftluo]

fix #3165

Can this PR also be patched to the 0.3.x version of tokio-util?

[Darksonn]

If you want this to be added to the v0.3.x version of tokio-util too, then please submit a backport PR as well. You will need to base your branch off the v0.2.x branch as tokio-util 0.3 is on the Tokio 0.2 branch.

[driftluo]

If you want this to be added to the v0.3.x version of tokio-util too, then please submit a backport PR as well. You will need to base your branch off the v0.2.x branch as tokio-util 0.3 is on the Tokio 0.2 branch.

I bypassed this problem with the new type and don't need a patch if the 0.3 util is not needed for release new version, like it:

pub(crate) struct PatchedReadPart {
    buffer: bytes::BytesMut,
    io: ReadHalf<io>,
}

impl AsyncRead for PatchedReadPart {
    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        if self.buffer.is_empty() {
            Pin::new(&mut self.io).poll_read(cx, buf)
        } else {
            let n = ::std::cmp::min(buf.len(), self.buffer.len());
            let b = self.buffer.split_to(n);
            buf[..n].copy_from_slice(&b);
            Poll::Ready(Ok(n))
        }
    }
}