don't force use to specify item route for an entity

sometimes a simple entity is not necessary to have an item operation,
sometimes an entity have two item operations ,one for admin ,one for customer. at this situation, it should let the developer to specify which one.

api-platform can handle the general case, but also should add the flexibility.

@dunglas ,are you willing to add API relation to api-platform please? I created issue[api relation](https://github.com/api-platform/api-platform/issues/778) a month ago? in my case API relation customization is very urgent.