[feat] engines: add GMX search engine (#5967)

Notes:
- Safesearch doesn't seem to work properly?
- In theory multiple languages are supported, but even in the web UI, they don't work properly
- Possibly, we could cache the request hashes (h query parameter), I'm not sure if it ever changes

searx/settings.yml

@@ -1036,6 +1036,11 @@ engines:
     shortcut: gitea
     disabled: true
 
+  - name: gmx
+    engine: gmx
+    shortcut: gmx
+    disabled: true
+
   - name: goodreads
     engine: goodreads
     shortcut: good