Added return_url and annotation methods

[sergunik]

I've added return_url validation parameter because need to have redirection after payment. Paddle API supports it.
Also Added annotation in GeneratePayLinkRequest class because my PHPStorm likes it :)
Cheers!

[pascalbaljet]

Thank you!

[brunocfalcao]

Hi,

If you check the PayLink reference, we can pass a custom attribute like {checkout_hash}, so if you use validation 'URL' it will give an error if I try to use that special Paddle parameter.

The (untested yet) way to make it is to use the urlencode like this:

            ->returnUrl(env('PADDLE_RETURN_URL').'/'.urlencode('{checkout_hash}'))

Maybe that validation should be removed or allowed to use {checkout_hash} ?

Thanks!
Bruno