Check that possibleTypes for apollo are correct

What does this MR do and why?

When our GraphQL Union types or Interfaces change, we need to regenerate app/assets/javascripts/graphql_shared/possibleTypes.json. Not doing this has caused a regression before, so we create a little script to regenerate the types and check whether the checked in types are correct in CI.

This would have help prevent: !82833 (merged)

Fixes: #351896 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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