Implement Association interface

Implement xyz/openbmc_project/Association/Definitions.interface in
activation and item_updater.

Tested: Verify the association is created after uploading a dummy
        tarball.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I217bddc48534a0b3b17359e0f3409f6c7c953f08
diff --git a/src/types.hpp b/src/types.hpp
new file mode 100644
index 0000000..9ba4383
--- /dev/null
+++ b/src/types.hpp
@@ -0,0 +1,18 @@
+#pragma once
+
+#include <string>
+#include <tuple>
+#include <vector>
+
+namespace phosphor
+{
+namespace software
+{
+namespace updater
+{
+
+using AssociationList =
+    std::vector<std::tuple<std::string, std::string, std::string>>;
+}
+} // namespace software
+} // namespace phosphor