Add a generic event class for each category

[rmouritzen-splunk]

A generic event class for each category is helpful when mapping existing event logs to OCSF in situations where the category of an event is clear but the work to fully map events is not yet complete.

The alternative is loss of fidelity or just dropping the raw event (not mapping it at all while running in production), neither of which is desirable. The loss of fidelity arises from the use of the Base Event class instead of a generic category-specific class, and thus losing both the category and activity IDs for the generic class.

There should be a warning in the event class descriptions of these generic classes saying exactly why they exist, and steering people away from their use a final solution. The same warning should be added to Base Event.