Add logic for assembling config items from their parts

Previously, this was implemented in our invenio.cfg but more flaky because it had to deal with parsing and setting of environment variables. Implementing it as finalize_app hook instead allows us to rely on the fact that the environment variables have already been processed.

invenio_config_tuw/startup/__init__.py

+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2025 TU Wien.
+#
+# Invenio-Config-TUW is free software; you can redistribute it and/or modify
+# it under the terms of the MIT License; see LICENSE file for more details.
+
+"""Invenio-Config-TUW hacks and overrides to be applied on application startup.
+
+This module provides a blueprint whose sole purpose is to execute some code exactly
+once during application startup (via ``bp.record_once()``).
+These functions will be executed after the Invenio modules' extensions have been
+initialized, and thus we can rely on them being already available.
+"""
+
+
+from .config import finalize_config, patch_flask_create_url_adapter
+from .misc import (
+    customize_curation_request_type,
+    override_search_drafts_options,
+    register_menu_entries,
+    register_smtp_error_handler,
+)
+
+__all__ = (
+    "customize_curation_request_type",
+    "finalize_config",
+    "override_search_drafts_options",
+    "patch_flask_create_url_adapter",
+    "register_menu_entries",
+    "register_smtp_error_handler",
+)