Implements SDK callback for GameServer updates

This implements the gRPC system to send messages up to a unidirectional
stream from the sdk-server to the SDK.

This has been implemented in the Go, C++ and REST SDK.

Closes googleforgames#277

sdks/cpp/README.md

@@ -117,6 +117,25 @@ sdk->WatchGameServer([](stable::agones::dev::sdk::GameServer gameserver){
 For more information, you can also read the [SDK Overview](../), check out [sdk.h](sdk.h) and also look at the
 [C++ example](../../examples/cpp-simple).
 
+To get updates on the [backing `GameServer`](../README.md#gameserver) as they happen, 
+call `sdk->WatchGameServer([](stable::agones::dev::sdk::GameServer gameserver){...})`.
+
+⚠️⚠️⚠️ **`WatchGameServer` is currently a development feature and has not been released** ⚠️⚠️️⚠️ 
+
+This will call the passed in `std::function`
+synchronously (this is a blocking function, so you may want to run it in its own thread) whenever the backing `GameServer`
+is updated.
+
+```cpp
+sdk->WatchGameServer([](stable::agones::dev::sdk::GameServer gameserver){
+    std::cout << "GameServer Update, name: " << gameserver.object_meta().name() << std::endl;
+    std::cout << "GameServer Update, state: " << gameserver.status().state() << std::endl;
+});
+```
+
+For more information, you can also read the [SDK Overview](../), check out [sdk.h](sdk.h) and also look at the
+[C++ example](../../examples/cpp-simple).
+
 ### Failure
 When running on Agones, the above functions should only fail under exceptional circumstances, so please 
 file a bug if it occurs.

sdks/go/sdk.go

@@ -129,3 +129,33 @@ func (s *SDK) WatchGameServer(f GameServerCallback) error {
 	}()
 	return nil
 }
+
+// WatchGameServer asynchronously calls the given GameServerCallback with the current GameServer
+// configuration when the backing GameServer configuration is updated.
+// This function can be called multiple times to add more than one GameServerCallback.
+func (s *SDK) WatchGameServer(f GameServerCallback) error {
+	stream, err := s.client.WatchGameServer(s.ctx, &sdk.Empty{})
+	if err != nil {
+		return errors.Wrap(err, "could not watch gameserver")
+	}
+
+	go func() {
+		for {
+			var gs *sdk.GameServer
+			gs, err = stream.Recv()
+			if err != nil {
+				if err != io.EOF {
+					fmt.Printf("error watching GameServer: %s\n", err.Error())
+				} else {
+					fmt.Println("gameserver event stream EOF received")
+					return
+				}
+				// This is to wait for the reconnection, and not peg the CPU at 100%
+				time.Sleep(time.Second)
+				continue
+			}
+			f(gs)
+		}
+	}()
+	return nil
+}