[bug-5429]: Add isBlocking to input validator map

[OscarBakker]

how to test

• run backend and frontend with fetchQuestionsFromBackend to true (docker compose up -d in both signals and signals frontend)
• then add two questions where the alert is triggered on the first option of the first question:

• it should init the 'harde stop'

Ticket: SIG-5429

Signalen

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[vdegraaf]

LGTM