feat: add KeepAlive packet

NGINX really doesn't like it when the client doesn't send messages
for a while. This prevents it from closing the connection.

protobuf/2.0/Container.proto

@@ -1,8 +1,9 @@
+syntax = "proto3";
+
 import "./Authentication.proto";
 import "./Message.proto";
 import "./PublishDiscover.proto";
-
-syntax = "proto3";
+import "./KeepAlive.proto";
 
 // Client <-> Server
 message Container {
@@ -16,5 +17,6 @@ message Container {
         PublishResponse        PublishResponse        = 6;
         DiscoverRequest        DiscoverRequest        = 7;
         DiscoverResponse       DiscoverResponse       = 8;
+        KeepAlive              KeepAlive              = 9;
     }
 }

protobuf/2.0/KeepAlive.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+// Server <-> Client
+// data: any data, server ignores this
+message KeepAlive {
+    required uint32 data = 1;
+}