Add `event.category` to Asset Discovery

[JordanSh]

The event.category field is missing from Asset Discovery logs, which is essential for better event classification. This field is used by SIEM Readiness for categorizing logs.

The following allowed values should be used for the event.category field:

• api
• authentication
• configuration
• database
• driver
• email
• file
• host
• iam
• intrusion_detection
• library
• malware
• network
• package
• process
• registry
• session
• threat
• vulnerability
• web

optional: event.type and perhaps other essential fields should be considered as well