[TASK] Resolve hidden dependency from EXT:backend to EXT:rsaauth

Moves always-executed code into a hook which only executes
if the rsaauth extension is installed.

Change-Id: I11cb69d7d6447268e26c07f7dc02a8a15afe80da
Resolves: #80580
Releases: master
Reviewed-on: https://review.typo3.org/52269


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Kasper Ligaard <kasperligaard+typo3.org@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>