For generic interop with other parts of Qt, we need to be able to
extract the type-erased internal storage of QFuture<T>. In particular,
QtWebChannel needs this to transparently support QFuture<T> as a method
return type.
Task-number: QTBUG-92903
Change-Id: I763f054656b8810d58720262d364baf42c47eb37
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>