[FEATURE] Improve performance and usability while editing sys_filemounts

This patch combines the two fields base and path into a new field identifier.
In the backend UI, the new TCA type folder is used to select the folder.

This change improves the performance in the backend if the storage contains
a massive amount of folders and subfolders. Besides that, the usability is better
because of the browsing feature of the folder type.

Resolves: #97922
Related: #52374
Release: main
Change-Id: I3054ef98d31fd1615174d2fed68f639cf78f5f6c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75163


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>