refactor(dbus): Move D-Bus helper functions to a dedicated file

Consolidated `REVPI_DBUS_*` constants and `extend_interface` function
into `dbus_helper.py` for better modularity and reusability. Updated
imports across modules to reflect this change.

src/revpi_middleware/dbus_middleware1/__init__.py

@@ -2,27 +2,5 @@
 # SPDX-FileCopyrightText: 2025 KUNBUS GmbH
 # SPDX-License-Identifier: GPL-2.0-or-later
 """D-Bus middleware version 1 of revpi_middleware."""
-from ..__about__ import __author__, __copyright__, __license__, __version__
-
-REVPI_DBUS_NAME = "com.revolutionpi.middleware1"
-REVPI_DBUS_BASE_PATH = "/com/revolutionpi/middleware1"
-
-
-def extend_interface(*args) -> str:
-    """
-    Extends an interface name by appending additional segments to a pre-defined base name.
-
-    This function takes multiple arguments, concatenates them with a predefined base
-    interface name, and returns the resulting string, effectively constructing an
-    extended interface name.
-
-    Args:
-        *args: str
-            Components to be appended to the base interface name.
-
-    Returns:
-        str
-            Fully constructed interface name by joining the base interface name with
-            the provided segments.
-    """
-    return ".".join([REVPI_DBUS_NAME, *args])
+from .dbus_helper import REVPI_DBUS_BASE_PATH, REVPI_DBUS_NAME
+from .dbus_helper import extend_interface