Many people struggle to properly configure CORS handling when creating an API with Symfony 4. To solve this problem, this PR adds NelmioCors (that now has a dedicated Flex recipe) to the pack.

Thanks to the new Flex's unpack feature, adding an extra dependency in the pack isn't a big deal anymore, because the user can easily remove it if he wants to customize the installation.

Another advantage of this PR: it will reduce the gap between the standard API Platform distribution and composer req api.