Fixes #8514 - Add support for requests to smart proxy discovery plugin

> @lzap when I remove the "Concerns" namespace and change it in the engine.rb I get:
> ```uninitialized constant DiscoverySubnet (NameError)```

Oh right, I have added concerns subdir to autoload paths in my own patch
that was not yet merged. Let's leave this as is and we can refactor
later on.

-- 
Later,
 Lukas #lzap Zapletal