Add support for array query parameters

[rapkis]

I've added a feature which allows to generate URLs with array query parameters. URLs often have such parameters, so I think this feature is useful for this package.

The changes may be a bit controversial so I'm open for feedback. I think that using native PHP functions is better, since the implementation problem is already solved in them, and there's no longer a need for the Arr helper class.
I'm not sure why these functions weren't used previously, so if there was a reason - I'd appreciate some feedback here.
The only downside I see with parse_str is this (details):

variables in PHP can't have dots and spaces in their names, those are converted to underscores

Main changes:

• use native http_build_query function to cast QueryParameterBag to string
• use native parse_str function to parse input in QueryParameterBag fromString() method
• remove helper class as it is no longer used
• use union types for the set() method, in order to clearly support string and array only
• fix unit tests to cover new feature

Whatever implementation fits best - I'm mostly interested in the array support itself. If there's any questions regarding it - please let me know.

[AidasK]

Can we merge this? Looks like a good patch

[freekmurze]

Thanks!