Improve DNS plug-in restart (#5999)

* Improve DNS plug-in restart

Instead of simply go by PrimaryConnectioon change, use the DnsManager
Configuration property. This property is ultimately used to write the
DNS plug-in configuration, so it is really the relevant information
we pass on to the plug-in.

* Check for changes and restart DNS plugin

* Check for changes in plug-in DNS

Cache last local (NetworkManager) provided DNS servers. Check against
this DNS server list when deciding when to restart the DNS plug-in.

* Check connectivity unthrottled in certain situations

* Fix pytest

* Fix pytest

* Improve test coverage for DNS plugins restart functionality

* Apply suggestions from code review

Co-authored-by: Mike Degatano <michael.degatano@gmail.com>

* Debounce local DNS changes and event based connectivity checks

* Remove connection check logic

* Remove unthrottled connectivity check

* Fix delayed call

* Store restart task and cancel in case a restart is running

* Improve DNS configuration change tests

* Remove stale code

* Improve DNS plug-in tests, less mocking

* Cover multiple private functions at once

Improve tests around notify_locals_changed() to cover multiple
functions at once.

---------

Co-authored-by: Mike Degatano <michael.degatano@gmail.com>

tests/conftest.py

@@ -66,6 +66,7 @@ from .dbus_service_mocks.base import DBusServiceMock
 from .dbus_service_mocks.network_connection_settings import (
     ConnectionSettings as ConnectionSettingsService,
 )
+from .dbus_service_mocks.network_dns_manager import DnsManager as DnsManagerService
 from .dbus_service_mocks.network_manager import NetworkManager as NetworkManagerService
 
 # pylint: disable=redefined-outer-name, protected-access
@@ -220,6 +221,14 @@ async def network_manager_service(
     yield network_manager_services["network_manager"]
 
 
+@pytest.fixture
+async def dns_manager_service(
+    network_manager_services: dict[str, DBusServiceMock | dict[str, DBusServiceMock]],
+) -> AsyncGenerator[DnsManagerService]:
+    """Return DNS Manager service mock."""
+    yield network_manager_services["network_dns_manager"]
+
+
 @pytest.fixture(name="connection_settings_service")
 async def fixture_connection_settings_service(
     network_manager_services: dict[str, DBusServiceMock | dict[str, DBusServiceMock]],