Add axum_core::body::Body

[davidpdrsn]

This is part of the work towards 0.7.0

Hyper 1.0 is moving hyper::Body to hyper-util. That crate isn't stable so I don't think we should have it in axum's public API. This PR adds our own body type that just wraps another boxed body that we can use instead. It just adds the new type and doesn't actually use it yet. I'll do that in follow up PRs.

Generally my thinking is to replace Service impls with:

impl<B> Service<Request<B>> for Router<()>
where
    B: http_body::Body<Data = Bytes> + Send + 'static,
    B::Error: Into<BoxError>,
{
    fn call(&mut self, request: Request<B>) -> Self::Future {
        let request = request.map(|body| axum_core::Body::new(body));
        self.call_route(request)
    }

    // ...
}

So receive any boxable body but internally always use axum_core::Body. This means things don't need to be generic the request body which should be more ergonomic and fix issues such as #1110

axum_core::Body will also be re-exported at axum::body::Body for 0.7.0 but didn't do that here.