[Fleet] Add "last data received" field to agent details

[joshdover]

Today we currently show a timestamp labeled "Last activity" which indicates the last time an Agent checked in with the Fleet Server.

This timestamp can give a false sense of security that the agent is properly ingesting data and could hide network or configuration issues with the connection between the Agent and Elasticsearch. It would be helpful for troubleshooting if we also indicated the last time data was sent by the Agent to Elasticsearch.

This could be a simple query on all the metrics-*,logs-*,traces-*,synthetics-* data streams for the latest document for the given agent id. We should be careful about the performance of this query since it could hit many shards. The allow_partial_search_results and timeout search options may be helpful here.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)