[FEATURE] Support custom translation servers for extensions

Use of signal/slot to post-process the mirror URL when fetching
localization packages for a given extension key.

Resolves: #50467
Releases: 6.2
Change-Id: I6a0edc0a6765da05458c191f968f72068132b87b
Reviewed-on: https://review.typo3.org/22587
Reviewed-by: Stefan Neufeind
Reviewed-by: Begoña
Reviewed-by: Frans Saris
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers