don't let ExtensionApp jpserver_extensions be overridden by config

[minrk]

ensures jpserver_extensions set in make_serverapp is preserved after loading config files in ServerApp.initialize.

closes jupyterhub/jupyterhub#4859

Repro:

# jupyter_server_config.py
c.ServerApp.jpserver_extensions = {"anything": False}

# testext.py
from jupyter_server.extension.application import ExtensionApp


def _jupyter_server_extension_points():
    return [{"module": __name__, "app": TestExtension}]


class TestExtension(ExtensionApp):
    name = "tester"


if __name__ == "__main__":
    TestExtension.launch_instance()

and run:

python3 testext.py --config ./jupyter_server_config.py

which results in:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/minrk/dev/jpy/jupyterhub/test-env/testext.py", line 13, in <module>
    TestExtension.launch_instance()
  File "/Users/minrk/dev/jpy/jupyter_server/jupyter_server/extension/application.py", line 618, in launch_instance
    serverapp = cls.initialize_server(argv=args)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/minrk/dev/jpy/jupyter_server/jupyter_server/extension/application.py", line 587, in initialize_server
    serverapp.initialize(
  File "/Users/minrk/.virtualenvs/test-env/lib/python3.11/site-packages/traitlets/config/application.py", line 118, in inner
    return method(app, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/minrk/dev/jpy/jupyter_server/jupyter_server/serverapp.py", line 2750, in initialize
    point = self.extension_manager.extension_points[starter_extension]
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
KeyError: 'tester'

[Zsailer]

Thanks, @minrk!