Promote Splunk's Network File Activity event class to core schema

[pagbabian-splunk]

Splunk created this event class, in order to have network drive file system events (e.g. Box, OneDrive, Google Drive). The reason it is a separate class in the Network category is to impart the different semantics of a remote file transfer. An alternate way of doing this would have been to create a Network profile (in work) and apply it to the System Activity event class, File System Activity. However there are a few other activities, such as Upload, Download etc. This approach would not impart the semantics of the different activity as distinctly as having a event class with the semantics in the name.

The schema in question is found here:
https://github.com/ocsf/splunk/blob/main/events/network/file_activity.json

[mikeradka]

Completed via PR #501