feat(client): Make client an optional feature

cc #2223

BREAKING CHANGE: The HTTP client of hyper is now an optional feature. To
  enable the client, add `features = ["client"]` to the dependency in
  your `Cargo.toml`.

src/lib.rs

@@ -71,9 +71,14 @@ pub mod upgrade;
 cfg_any_http! {
     mod headers;
     mod proto;
-    pub mod client;
     pub mod server;
 
-    pub use crate::client::Client;
     pub use crate::server::Server;
 }
+
+cfg_feature! {
+    #![all(feature = "client", any(feature = "http1", feature = "http2"))]
+
+    pub mod client;
+    pub use crate::client::Client;
+}