add "deserialization" and "serialization" ES child spans for ES queries

[trentm]

The @elastic/elasticsearch client provides "serialization" and "deserialization" diagnostic events (https://www.elastic.co/guide/en/elasticsearch/client/javascript-api/current/observability.html#_events) that could be used to generate separate child events of an Elasticsearch query span.

This could be very useful for observing the possible impact of sync (de)serialization for large ES payloads.