Add the ability to disable HTTP/2

I have noticed that HTTP/2 tends to be unreliable in certain
circumstances due to bugs in Go's HTTP/2 library. See the following
upstream issues:

golang/go#36026
golang/go#59690

Let's add a configuration option here, so that HTTP/2 can be disabled on
an individual basis if needed.

pkg/http/configuration.go

@@ -22,7 +22,7 @@ func NewRoundTripperFromConfiguration(configuration *pb.ClientConfiguration) (ht
 			Timeout:   30 * time.Second,
 			KeepAlive: 30 * time.Second,
 		}).DialContext,
-		ForceAttemptHTTP2: true,
+		ForceAttemptHTTP2: !configuration.DisableHttp2,
 		TLSClientConfig:   tlsConfig,
 	}
 	if proxyURL := configuration.GetProxyUrl(); proxyURL != "" {

pkg/proto/configuration/http/http.proto

@@ -30,6 +30,17 @@ message ClientConfiguration {
   //
   //         "Basic " + std.base64(username + ":" + password)
   repeated HeaderValues add_headers = 5;
+
+  // Use HTTP/1.1, even if the server supports HTTP/2.
+  //
+  // This option can be used to work around HTTP client or server bugs.
+  // For example, the following bug in Go's HTTP/2 client may cause
+  // requests to be sent across network connections that are known to be
+  // unresponsive, leading to request timeouts:
+  //
+  // https://github.com/golang/go/issues/36026
+  // https://github.com/golang/go/issues/59690
+  bool disable_http2 = 6;
 }
 
 message ServerConfiguration {