[refactor] migrate plugins from "module" to class SXNGPlugin

This patch brings two major changes:

- ``Result.filter_urls(..)`` to pass a filter function for URL fields
- The ``enabled_plugins:`` section in SearXNG's settings do no longer exists.

To understand plugin development compile documentation:

    $ make docs.clean docs.live

and read http://0.0.0.0:8000/dev/plugins/development.html

There is no longer a distinction between built-in and external plugin, all
plugins are registered via the settings in the ``plugins:`` section.

In SearXNG, plugins can be registered via a fully qualified class name.  A
configuration (`PluginCfg`) can be transferred to the plugin, e.g. to activate
it by default / *opt-in* or *opt-out* from user's point of view.

built-in plugins
================

The built-in plugins are all located in the namespace `searx.plugins`.

.. code:: yaml

    plugins:

      searx.plugins.calculator.SXNGPlugin:
        active: true

      searx.plugins.hash_plugin.SXNGPlugin:
        active: true

      searx.plugins.self_info.SXNGPlugin:
        active: true

      searx.plugins.tracker_url_remover.SXNGPlugin:
        active: true

      searx.plugins.unit_converter.SXNGPlugin:
        active: true

      searx.plugins.ahmia_filter.SXNGPlugin:
        active: true

      searx.plugins.hostnames.SXNGPlugin:
        active: true

      searx.plugins.oa_doi_rewrite.SXNGPlugin:
        active: false

      searx.plugins.tor_check.SXNGPlugin:
        active: false

external plugins
================

SearXNG supports *external plugins* / there is no need to install one, SearXNG
runs out of the box.

- Only show green hosted results: https://github.com/return42/tgwf-searx-plugins/

To get a developer installation in a SearXNG developer environment:

.. code:: sh

   $ git clone git@github.com:return42/tgwf-searx-plugins.git
   $ ./manage pyenv.cmd python -m \
         pip install -e tgwf-searx-plugins

To register the plugin in SearXNG add ``only_show_green_results.SXNGPlugin`` to
the ``plugins:``:

.. code:: yaml

    plugins:
      # ...
      only_show_green_results.SXNGPlugin:
        active: false

Result.filter_urls(..)
======================

The ``Result.filter_urls(..)`` can be used to filter and/or modify URL fields.
In the following example, the filter function ``my_url_filter``:

.. code:: python

   def my_url_filter(result, field_name, url_src) -> bool | str:
       if "google" in url_src:
           return False              # remove URL field from result
       if "facebook" in url_src:
           new_url = url_src.replace("facebook", "fb-dummy")
           return new_url            # return modified URL
       return True                   # leave URL in field unchanged

is applied to all URL fields in the :py:obj:`Plugin.on_result` hook:

.. code:: python

   class MyUrlFilter(Plugin):
       ...
       def on_result(self, request, search, result) -> bool:
           result.filter_urls(my_url_filter)
           return True

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/plugins/unit_converter.py

@@ -7,36 +7,74 @@ converters, each converter is one item in the list (compare
 :py:obj:`ADDITIONAL_UNITS`).  If the symbols are ambiguous, the matching units
 of measurement are evaluated.  The weighting in the evaluation results from the
 sorting of the :py:obj:`list of unit converters<symbol_to_si>`.
-
-Enable in ``settings.yml``:
-
-.. code:: yaml
-
-  enabled_plugins:
-    ..
-    - 'Unit converter plugin'
-
 """
-
 from __future__ import annotations
+import typing
 import re
 import babel.numbers
 
 from flask_babel import gettext, get_locale
 
 from searx import data
+from searx.plugins import Plugin, PluginInfo
 from searx.result_types import EngineResults
 
+if typing.TYPE_CHECKING:
+    from searx.search import SearchWithPlugins
+    from searx.extended_types import SXNG_Request
+    from searx.plugins import PluginCfg
 
-name = "Unit converter plugin"
-description = gettext("Convert between units")
-default_on = True
 
-plugin_id = "unit_converter"
-preference_section = "general"
+name = ""
+description = gettext("")
+
+plugin_id = ""
+preference_section = ""
 
 CONVERT_KEYWORDS = ["in", "to", "as"]
 
+
+class SXNGPlugin(Plugin):
+    """Convert between units.  The result is displayed in area for the
+    "answers".
+    """
+
+    id = "unit_converter"
+
+    def __init__(self, plg_cfg: "PluginCfg") -> None:
+        super().__init__(plg_cfg)
+
+        self.info = PluginInfo(
+            id=self.id,
+            name=gettext("Unit converter plugin"),
+            description=gettext("Convert between units"),
+            preference_section="general",
+        )
+
+    def post_search(self, request: "SXNG_Request", search: "SearchWithPlugins") -> EngineResults:
+        results = EngineResults()
+
+        # only convert between units on the first page
+        if search.search_query.pageno > 1:
+            return results
+
+        query = search.search_query.query
+        query_parts = query.split(" ")
+
+        if len(query_parts) < 3:
+            return results
+
+        for query_part in query_parts:
+            for keyword in CONVERT_KEYWORDS:
+                if query_part == keyword:
+                    from_query, to_query = query.split(keyword, 1)
+                    target_val = _parse_text_and_convert(from_query.strip(), to_query.strip())
+                    if target_val:
+                        results.add(results.types.Answer(answer=target_val))
+
+        return results
+
+
 # inspired from https://stackoverflow.com/a/42475086
 RE_MEASURE = r'''
 (?P<sign>[-+]?)         # +/- or nothing for positive
@@ -243,27 +281,3 @@ def _parse_text_and_convert(from_query, to_query) -> str | None:
         result = babel.numbers.format_decimal(value, locale=_locale, format='#,##0.##########;-#')
 
     return f'{result} {target_symbol}'
-
-
-def post_search(_request, search) -> EngineResults:
-    results = EngineResults()
-
-    # only convert between units on the first page
-    if search.search_query.pageno > 1:
-        return results
-
-    query = search.search_query.query
-    query_parts = query.split(" ")
-
-    if len(query_parts) < 3:
-        return results
-
-    for query_part in query_parts:
-        for keyword in CONVERT_KEYWORDS:
-            if query_part == keyword:
-                from_query, to_query = query.split(keyword, 1)
-                target_val = _parse_text_and_convert(from_query.strip(), to_query.strip())
-                if target_val:
-                    results.add(results.types.Answer(answer=target_val))
-
-    return results