feat(client): expose connect types without proto feature (#2377)

Make it possible to refer to Connected, Connection, HttpConnector, etc.
without enabling either of the http1/http2 features. This makes feature
selection work better for downstream libraries like hyper-openssl, which
don't want to commit to any particular protocol.

Fix #2376.

src/client/connect/http.rs

@@ -27,6 +27,7 @@ use super::{Connected, Connection};
 ///
 /// Sets the [`HttpInfo`](HttpInfo) value on responses, which includes
 /// transport information such as the remote socket address used.
+#[cfg_attr(docsrs, doc(cfg(feature = "tcp")))]
 #[derive(Clone)]
 pub struct HttpConnector<R = GaiResolver> {
     config: Arc<Config>,