[FEATURE] Signal to process actions in Extension Manager

This adds a way for third party extensions to process
extensions' available actions in Extension Manager.

Resolves: #56869
Releases: 6.2
Change-Id: I7cbacba7fccfd3178dc1265ad69bf5dec531cf70
Reviewed-on: https://review.typo3.org/28347
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers