[refactor] typification of SearXNG / EngineResults

In [1] and [2] we discussed the need of a Result.results property and how we can
avoid unclear code.  This patch implements a class for the reslut-lists of
engines::

    searx.result_types.EngineResults

A simple example for the usage in engine development::

    from searx.result_types import EngineResults
    ...
    def response(resp) -> EngineResults:
        res = EngineResults()
        ...
        res.add( res.types.Answer(answer="lorem ipsum ..", url="https://example.org") )
        ...
        return res

[1] https://github.com/searxng/searxng/pull/4183#pullrequestreview-257400034
[2] https://github.com/searxng/searxng/pull/4183#issuecomment-2614301580
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/plugins/hash_plugin.py

@@ -9,7 +9,7 @@ import hashlib
 from flask_babel import gettext
 
 from searx.plugins import Plugin, PluginInfo
-from searx.result_types import Answer
+from searx.result_types import EngineResults
 
 if typing.TYPE_CHECKING:
     from searx.search import SearchWithPlugins
@@ -37,9 +37,9 @@ class SXNGPlugin(Plugin):
             preference_section="query",
         )
 
-    def post_search(self, request: "SXNG_Request", search: "SearchWithPlugins") -> list[Answer]:
+    def post_search(self, request: "SXNG_Request", search: "SearchWithPlugins") -> EngineResults:
         """Returns a result list only for the first page."""
-        results = []
+        results = EngineResults()
 
         if search.search_query.pageno > 1:
             return results
@@ -61,6 +61,6 @@ class SXNGPlugin(Plugin):
         f.update(string.encode("utf-8").strip())
         answer = function + " " + gettext("hash digest") + ": " + f.hexdigest()
 
-        Answer(results=results, answer=answer)
+        results.add(results.types.Answer(answer=answer))
 
         return results