[TASK] Add a system check for external downloads

This patch adds a check to the SystemEnvironment checks to test
whether any method for retrieving external files is available.
This involves allow_url_fopen PHP setting and the usage of curl.
Either of these two must be available otherwise any download
(e.g. from TER) fails.

Resolves: #58316
Releases: 6.2
Change-Id: I7dd7d2d7d2ce102723af3315f4ad4342610b4147
Reviewed-on: https://review.typo3.org/30486
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Fabien Udriot
Reviewed-by: Marcin Sągol
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers