[mod] limiter: trusted proxies (#4911)

Replaces `x_for` functionality with `trusted_proxies`. This allows defining
which IP / ranges to trust extracting the client IP address from X-Forwarded-For
and X-Real-IP headers.

We don't know if the proxy chain will give us the proper client
address (REMOTE_ADDR in the WSGI environment), so we rely on reading the headers
of the proxy before SearXNG (if there is one, in that case it must be added to
trusted_proxies) hoping it has done the proper checks. In case a proxy in the
chain does not check the client address correctly, integrity is compromised and
this should be fixed by whoever manages the proxy, not us.

Closes:

- https://github.com/searxng/searxng/issues/4940
- https://github.com/searxng/searxng/issues/4939
- https://github.com/searxng/searxng/issues/4907
- https://github.com/searxng/searxng/issues/3632
- https://github.com/searxng/searxng/issues/3191
- https://github.com/searxng/searxng/issues/1237

Related:

- https://github.com/searxng/searxng-docker/issues/386
- https://github.com/inetol-infrastructure/searxng-container/issues/81

searx/botdetection/valkeydb.py

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Providing a Valkey database for the botdetection methods."""
+
+from __future__ import annotations
+
+import valkey
+
+__all__ = ["set_valkey_client", "get_valkey_client"]
+
+CLIENT: valkey.Valkey | None = None
+"""Global Valkey DB connection (Valkey client object)."""
+
+
+def set_valkey_client(valkey_client: valkey.Valkey):
+    global CLIENT  # pylint: disable=global-statement
+    CLIENT = valkey_client
+
+
+def get_valkey_client() -> valkey.Valkey:
+    if CLIENT is None:
+        raise ValueError("No connection to the Valkey database has been established.")
+    return CLIENT