Add framing for `TcpSocket`

Hi, I would like to add framing to WASI TCP streams. Tokio provides this functionality with `tokio_util::codec::{FramedRead, FramedWrite}`, which relies on `tokio::io::{AsyncRead, AsyncWrite}`. Would it be possible to implement these traits for `wit_bindgen::async_support::{StreamReader, StreamWriter}` or `TcpSocket`?

I think it would look like this:

```rs
impl tokio::io::AsyncRead for wit_bindgen::async_support::StreamWriter {
fn poll_read(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
buf: &mut tokio::io::ReadBuf<'_>,
) -> std::task::Poll> {
...
}
}

impl tokio::io::AsyncWrite for wit_bindgen::async_support::StreamWriter {
fn poll_write(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
buf: &[u8],
) -> std::task::Poll> {
...
}

fn poll_flush(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
) -> std::task::Poll> {
...
}

fn poll_shutdown(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
) -> std::task::Poll> {
...
}
}
```