Make pytest plugin optional and provide it in a separate extra requires

This allows tests that require the pytest plugin to depend on swh.journal[pytest] rather than the full swh.journal[testing]. It also avoids importing the whole kafka suite as soon as swh.journal is installed.

conftest.py

+# Copyright (C) 2025 The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+pytest_plugins = ["swh.journal.pytest_plugin"]