add dropoff event to ObjectTracing

[cfkoehler]

To test need to run like: LOG_OBJECT_TRACE=true ./emissary server

example output:

{"@timestamp":"2025-02-08T21:07:19.913Z","stage":"PICK_UP","inputFileName":"emissary-knight.png","node":"localhost"}
{"@timestamp":"2025-02-08T21:07:19.975Z","stage":"DROP_OFF","outputFileName":"20250392107_44b35a39-9b85-4752-ad78-fd9f46c69d48_localhost_json","inputFileName":"emissary-knight.png","outputType":"JSON","uuid":"437a7e20-0063-43a0-8d2d-9c781e26fa7c","node":"localhost"}
{"@timestamp":"2025-02-08T21:07:19.997Z","stage":"DROP_OFF","outputFileName":"202502082107000localhost.xml","inputFileName":"emissary-knight.png","outputType":"XML","uuid":"437a7e20-0063-43a0-8d2d-9c781e26fa7c","node":"localhost"}