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.

src/nominatim_api/typing.py

@@ -21,9 +21,11 @@ if TYPE_CHECKING:
     from typing import Any
     import sqlalchemy as sa
     import os
-    from typing_extensions import (TypeAlias as TypeAlias)
+    from typing_extensions import (TypeAlias as TypeAlias,
+                                   Protocol as Protocol)
 else:
     TypeAlias = str
+    Protocol = object
 
 StrPath = Union[str, 'os.PathLike[str]']