[FEATURE] Add option to set an alternative domain for preview

Make it possible to set an alternative domain for preview with
TCEMAIN.viewDomain = example.com
in PageTS. It is useful for multidomain sites.

Resolves: #30889
Documentation: #30889
Releases: 6.2
Change-Id: If00dabcd8c3c32685bd89707ba60f31be26a6dca
Reviewed-on: https://review.typo3.org/24626
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind