Hyper 0.x server support

[goatgoose]

Problem:

s2n-tls-hyper provides hyper 1.x client compatibility. Hyper 1.x servers can use s2n-tls-tokio directly, and as such, no support is provided in s2n-tls-hyper.

However, hyper 0.x servers do require a compatibility layer in order to work with s2n-tls. We should provide support for this in s2n-tls-hyper, for applications that haven't yet migrated to hyper 1.x.

Solution:

@jmayclin implemented the following tls-listener adapter for s2n-tls, which allows s2n-tls to be used with the hyper 0.x server.

tls-listener adapter

// this struct is a new-type wrapper to allow a foreign trait to be implemented
// on the TlsAcceptor struct
#[derive(Clone)]
pub struct MyS2NAcceptor(s2n_tls_tokio::TlsAcceptor);

impl MyS2NAcceptor {
    /// create an S2NAcceptor from an S2N Config.
    pub fn new(config: s2n_tls::config::Config) -> Self {
        let acceptor = s2n_tls_tokio::TlsAcceptor::new(config);
        MyS2NAcceptor(acceptor)
    }
}

// implement the tls_listener::AsyncTls trait over a connection type C, for MyS2NTokioAcceptor
impl tls_listener::AsyncTls for MyS2NAcceptor {
    type Stream = s2n_tls_tokio::TlsStream;
    type Error = s2n_tls::error::Error;

    type AcceptFuture = Pin> + Send>>;

    fn accept(&self, stream: C) -> Self::AcceptFuture {
        let tls = self.clone();
        Box::pin(async move { s2n_tls_tokio::TlsAcceptor::accept(&tls.0, stream).await })
    }
}

We should add this tls-listener adapter to s2n-tls-hyper behind a feature flag.

Requirements / Acceptance Criteria:

In addition to this adapter, we should add a self-talk test that echos bytes to a client, similar to the existing hyper 1.x client self-talk tests.