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`.

Cargo.toml

@@ -74,11 +74,12 @@ pnet = "0.25.0"
 default = [
     "runtime",
     "stream",
-
+    "client",
     "http1",
     "http2",
 ]
 full = [
+    "client",
     "http1",
     "http2",
     "stream",
@@ -99,6 +100,8 @@ tcp = [
 http1 = []
 http2 = ["h2"]
 
+client = []
+
 # `impl Stream` for things
 stream = []