generalize normalization step for search query

It is now possible to configure functions for changing the query
input before it is analysed by the tokenizer.

Code is a cleaned-up version of the implementation by @miku.

test/python/api/query_processing/test_normalize.py

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2024 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+Tests for normalizing search queries.
+"""
+from pathlib import Path
+
+import pytest
+
+from icu import Transliterator
+
+import nominatim_api.search.query as qmod
+from nominatim_api.query_preprocessing.config import QueryConfig
+from nominatim_api.query_preprocessing import normalize
+
+def run_preprocessor_on(query, norm):
+    normalizer = Transliterator.createFromRules("normalization", norm)
+    proc = normalize.create(QueryConfig().set_normalizer(normalizer))
+
+    return proc(query)
+
+
+def test_normalize_simple():
+    norm = ':: lower();'
+    query = [qmod.Phrase(qmod.PhraseType.NONE, 'Hallo')]
+
+    out = run_preprocessor_on(query, norm)
+
+    assert len(out) == 1
+    assert out == [qmod.Phrase(qmod.PhraseType.NONE, 'hallo')]