[GR-69102] Use AbstractImageHeapList to restrict SubstrateDiagnostics.thunks runtime type.

[graalvmbot]

Introduce AbstractImageHeapList as a super type for HostedImageHeapList and RuntimeImageHeapList. This allows declaring a more restricted type for fields initialized with ImageHeapList#create(Class) to aid open-world analysis. When such a field is analysed in an open-world it gets injected all possible subtypes of its declared type. Declaring the type as AbstractImageHeapList allows us to only inject RuntimeImageHeapList.This enables a more precise analysis and avoids for example triggering false-positive violations of @RestrictHeapAccess: if the field was declared as List then implementations of List for which simple access operations can allocate, such as ArrayList#get(int), could become reachable from code annotated with @RestrictHeapAccess.