[FEATURE] Add hook in Typo3PageIndexer

In order to have a clean entry point for solrfal a new hook is introduced.

Change-Id: I6e702d7c4259b2c50949202ddf20f1705a673a25
Resolves: #58012
Releases: 3.0
Reviewed-on: https://review.typo3.org/29545
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner