feat(transport): provide generic access to connect info

[davidpdrsn]

Provides generic access to "connection info" provided by the stream of incoming connections through request extensions.

The Connected trait is changed to:

/// Trait that connected IO resources implement and use to produce info about the connection.
///
/// [...]
pub trait Connected {
    /// The connection info type the IO resources generates.
    type ConnectInfo: Clone + Send + Sync + 'static;

    /// Create type holding information about the connection.
    fn connect_info(&self) -> Self::ConnectInfo;
}

TcpStream and AddrStream implement this and return TcpConnectInfo which has remote_addr: Option<SocketAddr>. TlsStream implements it by returning TlsConnectInfo<T> where T is some other connect info type. That has get_ref to access the inner type and peer_certs to get the certificates.

ServerIo is then changed to no longer box the IO type but instead be generic such that no type information is erased:

pub(crate) enum ServerIo<IO> {
    Io(IO),
    #[cfg(feature = "tls")]
    TlsIo(TlsStream<IO>),
}

Where IO and TlsStream<IO> implement Connected.

In MakeSvc we access the connect info from the IO object and set it as a request extension so its accessible later.

Should be fairly flexible since we don't have to keep adding methods to the Connected trait but can instead add those on the types produced.

It is slightly less discoverable but I've updated the uds example to demo it and the remote_addr and peer_certs methods still exist on Request.

Fixes #365

[LucioFranco]

This is slick, love it!