Add the ability to extend the SObject handler classes

[jongpie]

There should be a way (at least in Apex - potentially Flow as well) to run additional logic in the LogHandler, LogEntryHandler and LogEntryEventHandler classes without having to modify these classes directly.

• Each handler class should manage running any extra logic for its SObject Type
• Developers should be able to write their own Apex classes to further manipulate the logging records in Log__c and LogEntry__c, LogEntryEvent__e

[jamessimone]

This is implied, but to make it explicit - all defined handlers should run after the base logging handlers so that they can hook into all fields updated before insert/update