feat: Add dummy main application script

The script uses the logger system and prints an string. You can try
the programm call with and without `-vv`so see the differences of
logging levels.

src/revpi_middleware/main_application.py

@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+# SPDX-FileCopyrightText: 2025 KUNBUS GmbH
+# SPDX-License-Identifier: GPL-2.0-or-later
+"""Main application of revpi-middleware daemon."""
+from logging import getLogger
+
+from . import proginit as pi
+
+log = getLogger(__name__)
+
+
+def main() -> int:
+    """
+    This is the main entry point.
+
+    We do not pack this to a 'if __name__ == "__main__"' thing to be able to
+    call this function from different places. The __main__ module and entry
+    points in the setup.py will call this function and linke to have an int
+    as return value for the exit code.
+    """
+    log.debug("Enter main() function")
+
+    log.warning(f"This is the empty '{pi.programname}' application so far")
+
+    log.debug("Leave main() function")
+
+    # Finally, call the cleanup function of proginit to flush all log buffers.
+    pi.cleanup()
+
+    return 0