[mod] limiter -> botdetection: modularization and documentation

In order to be able to meet the outstanding requirements, the implementation is
modularized and supplemented with documentation.

This patch does not contain functional change, except it fixes issue #2455

----

Aktivate limiter in the settings.yml and simulate a bot request by::

    curl -H 'Accept-Language: de-DE,en-US;q=0.7,en;q=0.3' \
         -H 'Accept: text/html'
         -H 'User-Agent: xyz' \
         -H 'Accept-Encoding: gzip' \
         'http://127.0.0.1:8888/search?q=foo'

In the LOG:

    DEBUG   searx.botdetection.link_token : missing ping for this request: .....

Since ``BURST_MAX_SUSPICIOUS = 2`` you can repeat the query above two time
before you get a "Too Many Requests" response.

Closes: https://github.com/searxng/searxng/issues/2455
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/botdetection/__init__.py

@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+""".. _botdetection src:
+
+Bot detection methods
+---------------------
+
+The methods implemented in this python package are use by the :ref:`limiter src`.
+
+"""
+
+import flask
+
+
+def dump_request(request: flask.Request):
+    return (
+        "%s: '%s'" % (request.headers.get('X-Forwarded-For'), request.path)
+        + " || form: %s" % request.form
+        + " || Accept: %s" % request.headers.get('Accept')
+        + " || Accept-Language: %s" % request.headers.get('Accept-Language')
+        + " || Accept-Encoding: %s" % request.headers.get('Accept-Encoding')
+        + " || Content-Type: %s" % request.headers.get('Content-Type')
+        + " || Content-Length: %s" % request.headers.get('Content-Length')
+        + " || Connection: %s" % request.headers.get('Connection')
+        + " || User-Agent: %s" % request.headers.get('User-Agent')
+    )