Adding public next_page() to Iterator.

The previous implementation may catch users off guard since
the iterator.page access may also update the value before
access.

In addition, this PR removed the _update_page() / next_page()
subclass behavior in _BlobIterator. Over-riding that method
was never intended. Instead makes a non-public class attribute
_PAGE_CLASS that can be replaced with Page subclasses. This
can be revisited if more implementations require custom
behavior on Page creation / Page.__init__.

packages/google-cloud-resource-manager/unit_tests/test_client.py

@@ -68,6 +68,7 @@ def dummy_response():
         iterator = self._makeOne(client)
         iterator._get_next_page_response = dummy_response
 
+        iterator.next_page()
         page = iterator.page
         self.assertEqual(page.num_items, 1)
         project = iterator.next()