feat(dbus): Add import for BusProvider in dbus_middleware1 module

This change includes the BusProvider import in the `__init__.py` file of
dbus_middleware1. It ensures the module has access to BusProvider
functionality, improving modularity and readiness for use.

src/revpi_middleware/dbus_middleware1/__init__.py

@@ -4,3 +4,5 @@
 """D-Bus middleware version 1 of revpi_middleware."""
 from .dbus_helper import REVPI_DBUS_BASE_PATH, REVPI_DBUS_NAME
 from .dbus_helper import extend_interface
+
+from .bus_provider import BusProvider