Add a way to map custom fields between LogEntryEvent__e and the custom objects

[jongpie]

Currently, plugins don't have an easy way to map field values from the LogEntryEvent__c object to the custom objects (Log__c, LogEntry__c, LogEntryTag__c, LoggerTag__c, and LoggerScenario__c). It can be done in Apex in a clunky way, but Nebula Logger doesn't provide an explicit mapping between LogEntryEvent__e records and their corresponding, transformed records stored in the custom objects. This could be solved with either a declarative way to map fields (preferred) or way in Apex for plugins to know the mapping between records.