Move access to $nonRelationTables and $nonRelationFields into methods and introduce 2 signals
The $nonRelationTables and $nonRelationFields in TYPO3\CMS\Core\Database\ReferenceIndex contains
tables and columns that the ReferenceIndexer should never process.
In order to extend this functionality with additional tables and columns it would be nice to wrap the access
to these 2 arrays in the methods and provided 2 additional signals for extending this behaviour.
This isssue provides a way to fix the 2 related issues.