FMassEntityManager::GetOrMakeCreationContext

The main use-case for this function is to create a blank [FEntityCreationContext](API\Runtime\MassEntity\FMassEntityManager\FEntityCreationContext) and hold on to it while creating a bunch of entities (with multiple calls to BatchCreate* and/or BatchBuild*) and modifying them (with mutating batched API) while not causing multiple Observers to trigger.