Offer a `Peek` interface over `Upgraded` connections

[RaitoBezarius]

Is your feature request related to a problem? Please describe.

I'm implementing some code that tries to detect whether the contents of a hyper upgraded connection can be handled or should be forwarded, for this, I need to peek 4KB and decide. Even though Read seems to be more of an BufRead, hyper do not expose any peeking methods on its usual APIs, so I would need to implement my own buffering, ending up with double buffers.

Describe the solution you'd like

• Add a Peek trait, implement it for Rewind<T>
• Implement it for Upgraded, H2Upgraded
• Implement compat with AsyncReadBuf for tokio.

Describe alternatives you've considered

• Do the buffering myself.

Additional context

I have some prototype of this feature modulo the tests, the only controversial aspect is, this is my Peek trait:

pub trait Peek {
    # Compared to all the other APIs, this returns a borrow of self. This should be fine lifetime-wise.
    fn poll_peek(self: Pin<&mut Self>, cx: &mut Context<'_>)
        -> Poll<Result<&[u8], std::io::Error>>;

    fn consume(self: Pin<&mut Self>, amt: usize);
}

If you are interested, I can try to finish my tests and PR this up.