[FEATURE] Add public API to get ApplicationContext

Application Context is initialized in Bootstrap. We need a
public API to get the current context without using the bootstrap
itself.

In Flow, the ObjectManager provides this API, but in TYPO3 CMS we
do not have this kind of ObjectManager (except in extbase).

The API call is:
\TYPO3\CMS\Core\Utility\GeneralUtility::getContext();

Resolves: #50983
Releases: 6.2
Change-Id: Ic900a503b3e2bc184d2bfd43345e72180887ef18
Reviewed-on: https://review.typo3.org/23018
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel