Add property watches

Property watches cache DBus property values given an externally
supplied index of property names and paths, in an externally
supplied storage location.

Change-Id: I155081da88c3ab0e4f6a13b012fc9719203b1888
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/src/data_types.hpp b/src/data_types.hpp
index 653b7c3..af0ef68 100644
--- a/src/data_types.hpp
+++ b/src/data_types.hpp
@@ -69,6 +69,17 @@
                           std::string,
                           sdbusplus::message::variant<T >>;
 
+/** @brief Lookup index for properties . */
+// *INDENT-OFF*
+using PropertyIndex = TupleRefMap <
+        TupleOfRefs<
+            const std::string,
+            const std::string,
+            any_ns::any>,
+        const std::string,
+        const std::string,
+        const std::string >;
+// *INDENT-ON*
 } // namespace monitoring
 } // namespace dbus
 } // namespace phosphor