Adding possibility to return context keys for dynamically populate context.

In my case, I want to be able to populate my context dynamically so i've added this possibility.
The code will look something like this: 
        $ruler = new \Hoa\Ruler\Ruler();

        $rule  = 'group in ("customer", "guest") and points > 30';
        
        $rule =  \Hoa\Ruler\Ruler::interprete($rule);
        $contextKeys = $ruler->getContextKeys();
        
        //dynamically populate context based on context keys
        $context = getContext (new \Hoa\Ruler\Context(), $contextKeys);
        
        var_dump($ruler->assert($rule, $context));