[FEATURE] Add filemount property to backend users

This patch set adds the new extbase filemount model as property to the
backend user model.

Change-Id: I8ed292fa4c38794a13ca62ee85f7767e16dc80e5
Resolves: #39324
Releases: 6.1
Reviewed-on: https://review.typo3.org/13880
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Oliver Klee
Reviewed-by: Markus Günther
Tested-by: Markus Günther
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring