Add generic to PaginatorInterface

I'm wondering if we could improve PaginatorInterface, which currently has a `@return PaginationInterface`

We could add `@template T of mixed` in the interface phpdoc, and replace the "mixed" above with "T"

Start at PaginatorInterface and read its existing PHPDoc, especially the PaginationInterface return annotation. Define the generic type in the interface and use it in that return annotation, then verify that the interface documentation expresses the paginator item type without changing runtime behavior.

Written by the indexing model from the issue text.