[FEATURE] Support multiple starting points in selectTree

A new TCA config `treeConfig/startingPoints` is introduced for selectTree
and category to allow multiple "roots" for the rendered tree.

This new settings superseds and deprecates `treeConfig/rootUid` being
capable of rendering only one root node.

To be able to re-use `AbstractItemProviderparseSiteConfiguration()`
without the implicit quoting, the method is split into separate parts.

Resolves: #95037
Releases: master
Change-Id: I3361daeed10b0f1ac1f8fa879850f79f4a85e23a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70819


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>