Merge installer-types properly

Merging object lists in extra does work ok except for installer-types, because that's just a list of string. Let's add support for that as well, and then remove the workaround in FedMS root composer as well.

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mentioned in commit 3fb5358d

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unassigned @jurgenhaas

EN: This issue hasn't been active for 3 months and will be closed in 2 weeks, if it still doesn't see any updates.

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added Documentation label and removed dormant label

EN: This issue hasn't been active for 3 months and will be closed in 2 weeks, if it still doesn't see any updates.

DE: Dieses Issue ist seit 3 Monaten inaktiv und wird in 2 Wochen automatisch geschlossen, falls es sich nicht ändert.

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