Add return types to `PersistentCollectionTrait`

[GromNaN]

Feature Request

Q	A
New Feature	no
RFC	no
BC Break	yes

Summary

The next major version of doctrine/collection will add return types to the interface methods. We need to add this return types in the trait and the generated collection code in order to support it.

• PersistentCollectionTrait
• Generated code

Also, the trait should be @internal and the generated collection classes @final. For now, this need to be done in a BC compatible way.