[interactive_media_ads] Fixes AdEventTypes not triggering on iOS in release mode (#8918)
Because _delegate was an unused field, Dart would garbage collect the value in release mode. To prevent this, this adds the @pragma('vm:entry-point') to indicate that the value is an entry point for the platform side. See the comment above the field.
Fixes https://github.com/flutter/flutter/issues/163049
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