[TASK] Allow flushing of the internal StorageRepository cache

To explicitly refresh the internal cache of StorageRepository
when a file storage has been added/removed/updated, a public
method flush() has been added to the class.

Releases: master
Resolves: #93057
Change-Id: I514bc5c39f6749a171006968a8b513803c599f4d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/67097


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>