Improve the performance of ObjectFieldsAppendingMarker by removing need to convert
the object to a JsonNode tree prior to writing the tree.

Instead, now we use an UnwrappingBeanSerializer to write the object directly
to the JsonGenerator (without having to convert to JsonNodes first).

Now, the ObjectFieldsAppendingMarker is actually faster than the MapEntriesAppendingMarker
when you want to embed fields in the logstash event json.