entry: add AdditionalData2

Support metadata as dict property "AdditionalData2".

Tested: Created a new event and ensured property is created.  Reloaded
the daemon with an old event and ensured property exists and is
populated.

```
.AdditionalData2                            property  a{ss}     8 "READING_VALUE" "98.6" "SENSOR_NAME" "Inlet Temperature" "THRESHOLD_VALUE" "40.0" "UNITS" "xyz.openbmc_project.Sensor.Value.Unit.DegreesC" "_CODE_FILE" "../log_create_main.cpp" "_CODE_FUNC" "int generate_event(const std::string&, const nlohmann::json_abi_v3_11_2::json&)" "_CODE_LINE" "34" "_PID" "2239596" emits-change writable
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4f96370b9fb07664192e4fe09308cab170176213
diff --git a/test/serialization_test_properties.cpp b/test/serialization_test_properties.cpp
index 2c83f1e..d7d5455 100644
--- a/test/serialization_test_properties.cpp
+++ b/test/serialization_test_properties.cpp
@@ -36,6 +36,7 @@
     EXPECT_EQ(input->timestamp(), output->timestamp());
     EXPECT_EQ(input->message(), output->message());
     EXPECT_EQ(input->additionalData(), output->additionalData());
+    EXPECT_EQ(input->additionalData2(), output->additionalData2());
     EXPECT_EQ(input->resolved(), output->resolved());
     EXPECT_EQ(input->associations(), output->associations());
     EXPECT_EQ(input->version(), output->version());