[FEATURE] Add signal for pre proccesing records

To determine the content that needs to be analysed in the linkvalidator,
allow external hooks/signals to adjust the variables when needed.

Resolves: #52217
Releases: master
Change-Id: Ie887852f356beb9b5e95188d309276dffd2155b8
Reviewed-on: http://review.typo3.org/24276


Reviewed-by: Jesper Paardekooper <jesper.paardekooper@gmail.com>
Tested-by: Jesper Paardekooper <jesper.paardekooper@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>