[BUGFIX] Properly iterate over extensions in extension manager

When downloading or updating extension using extension manager, the
resulting extensions are now properly iterated using `Object.keys`.
This is necessary since the extensions are provided as objects instead
of simple arrays.

In addition, the relevant methods in ExtensionManagementService now
have proper type declarations and annotations.

Resolves: #101430
Releases: main, 12.4
Change-Id: I112d59326ed5cb788bad919a7b556f7ee95b2b41
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81953


Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>