Add SNMPV3 engine ID to snmptrap event metadata

[edmocosta]

To improve the observability of the snmptrap plugin, it would be nice to include the SNMPv3 incoming authoritative/context engine IDs in the event's metadata. It's still pending to check if SNMP4J exposes that information to the trap listener, and if this feature is feasible.

[robbavey]

Closed by #76