[Game] Single chat message when a card transforms (#4764)

Transform currently logs "X destroys Y." followed by "X creates token: Z."
because the server implements it as DestroyCard + CreateToken events.

- Add Context_TransformCard (protocol extension 1010) to the transform
  event batch and a unit test asserting the server attaches it
- Suppress the destroy/create pair and the migration un-attach lines in
  the game log, printing one "X transforms Y into Z (P/T)." message

libcockatrice_protocol/libcockatrice/protocol/pb/CMakeLists.txt

@@ -80,6 +80,7 @@ set(PROTO_FILES
     context_ping_changed.proto
     context_ready_start.proto
     context_set_sideboard_lock.proto
+    context_transform_card.proto
     context_undo_draw.proto
     developer_commands.proto
     event_add_to_list.proto

libcockatrice_protocol/libcockatrice/protocol/pb/context_transform_card.proto

@@ -0,0 +1,8 @@
+syntax = "proto2";
+import "game_event_context.proto";
+
+message Context_TransformCard {
+    extend GameEventContext {
+        optional Context_TransformCard ext = 1010;
+    }
+}

libcockatrice_protocol/libcockatrice/protocol/pb/game_event_context.proto

@@ -11,6 +11,7 @@ message GameEventContext {
         CONNECTION_STATE_CHANGED = 1007;
         SET_SIDEBOARD_LOCK = 1008;
         UNCONCEDE = 1009;
+        TRANSFORM_CARD = 1010;
     }
     extensions 100 to max;
 }