Add the ability to set custom fields once per transaction/on every LogEntryEvent__e record

[jongpie]

This idea came up today on SFXD with @surajp. In release v4.13.14, a new instance method was added - LogEntryEventBuilder.setField() - to set custom fields on LogEntryEvent__e:

• This is useful for fields that are entry-specific (i.e., the field ultimately is mapped to a custom field on LogEntry__c)
• This is... confusing for fields that are transaction specific (i.e., the field value ultimately is mapped to a custom field on Log__c).
  • Right now, setting the custom field on every entry would probably be the best way to ensure the Log__c field is correctly populated. But doing it on every entry in a transaction is very tedious / not a great developer experience

Ideally, developers should only have to set the transaction-level fields once per transaction. This should be handled with 2 changes:

1. New static method Logger.setField(SObjectField field, Object value): This will give Apex developers a method that can be called once per transaction for a field, and all generated LogEntryEvent__e records will have it set
2. Improve Log__c mappings in LogEntryEventHandler: Right now, the trigger handler creates a Log__c record the first time it finds a LogEntryEvent__e record with a specific transaction ID - and then skips the log-related code for any subsequent events with the same transaction ID. In some situations, some Log__c data could be loss, so I'd like to improve how it works
   • If event A has SomeLogField__c = null, and event B has SomeLogField__c = 'some value', then the generated Log__c could have a null value, if both events are published/processed in the same async transaction

[jongpie]

This has been released in v4.14.14! See PR #772 for more details.