[TASK] Signal for BackendUtility::getPagesTSconfig()

A signal is dispatched in BackendUtility::getPagesTSconfig()
before the include lines are calculated.

This enables to supply additional PageTS for each page
programmatically, even in multi-site environments.

Releases: 6.3, 6.2
Resolves: #59703
Change-Id: I1cc1188eebbf6dee769c64a654339837e0cdce59
Reviewed-on: https://review.typo3.org/31002
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes