[TASK] Add wrapper for GeneralUtility::getIndpEnv in ContentObjectRenderer

A wrapper function for GeneralUtility::getIndpEnv() is required
to mock these static calls in unit tests.

Resolves: #60549
Releases: 6.3, 6.2
Change-Id: I47b565f5a5bd9b2445847d38447c01cfa20954c7
Reviewed-on: http://review.typo3.org/31792
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>