Issue #1296: Add File Access Check event class

[rmouritzen-splunk]

Related Issue:

#1296

Description of changes:

dd a File Access Check class to the System category.

The Splunk private schema has this class, and it is so far there is no equivalent in the core schema. This event class is useful for the 5140 and 5145 Windows Event types.

Delete once you have confirmed the following:

1. Did you add a single line summary of changes to Unreleased section in the CHANGELOG.md file?
   TBD

[rmouritzen-splunk]

@query-jeremy : The ocsf-validator's "Metaschema Validator" is incorrectly failing here.