[enh] limiter plugin

can replace filtron:
* rate limite the number of request per IP and per (IP, User-Agent)
* block some bots

use Redis
data stored in Redis never contains the IP addresses, only HMAC using the secret_key

Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings.yml

@@ -43,6 +43,7 @@ server:
   port: 8888
   bind_address: "127.0.0.1"
   base_url: false  # Possible values: false or "https://example.org/location".
+  limiter: false  # rate limit the number of request on the instance, block some bots
 
   # If your instance owns a /etc/searxng/settings.yml file, then set the following
   # values there.