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.

NOTE

This PR include a line in CHANGELIST.md for PR #1291, which was merged before the CHANGELOG.md was updated.

[rmouritzen-splunk]

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

Edit: This was actually a real problem. The actor attribute is in the primary group in the shared base system activity class, and so changing its recommended attribute to optional is incorrect.

[floydtree]

This is will be discussed in the Jan 8th's System Activity call, converting to draft for now.