privetd: Implement privet/v3/commands/execute

Privetd forwards JSON with command to buffet. Buffet updates command and
returns JSON representation of the command.

BUG=brillo:428
TEST=unittest

Change-Id: I7bdd4a70dd94e3dc3b4a974c7ac734e44564ac6e
Reviewed-on: https://chromium-review.googlesource.com/262216
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/manager.h b/buffet/manager.h
index d61c6fa..f44e075 100644
--- a/buffet/manager.h
+++ b/buffet/manager.h
@@ -68,7 +68,7 @@
   // Handles calls to org.chromium.Buffet.Manager.GetState().
   bool GetState(chromeos::ErrorPtr* error, std::string* state) override;
   // Handles calls to org.chromium.Buffet.Manager.AddCommand().
-  void AddCommand(DBusMethodResponse<> response,
+  void AddCommand(DBusMethodResponse<std::string> response,
                   const std::string& json_command) override;
   // Handles calls to org.chromium.Buffet.Manager.GetCommand().
   void GetCommand(DBusMethodResponse<std::string> response,