Fix legacy modules check_username_for_spam (#18135)

Broke in #17916, as the signature inspection incorrectly looks at the
wrapper function. We fix this by setting the signature on the wrapper
function to that of the wrapped function via `@functools.wraps`.

changelog.d/18135.bugfix

@@ -0,0 +1 @@
+Fix user directory search when using a legacy module with a `check_username_for_spam` callback. Broke in v1.122.0.

synapse/module_api/callbacks/spamchecker_callbacks.py

@@ -19,6 +19,7 @@
 #
 #
 
+import functools
 import inspect
 import logging
 from typing import (
@@ -297,6 +298,7 @@ def wrapper(
                         "Bad signature for callback check_registration_for_spam",
                     )
 
+            @functools.wraps(wrapped_func)
             def run(*args: Any, **kwargs: Any) -> Awaitable:
                 # Assertion required because mypy can't prove we won't change `f`
                 # back to `None`. See