Use composition instead of inheritance

[rmouritzen-splunk]

The 1.x schema hierarchy is not perfect, and it can be argued that any object-oriented style inheritance tree could never model reality well. Instead, consider organizing the schema with composition, adding things to an event class (or whatever we will call event definitions) by composing smaller elements together.

This should lead to less confusion about where a particular event definition should go: it can go in a single "category" or multiple... Further, this should allow similar types of data to always end up in the same place in an OCSF structurally. That is, there should be fewer incidental differences between event definitions (classes).