Allow \SplFileInfo or \Stringable in "files" methods

[Jean-Beru]

It could be useful to allow \Stringable or \SplFileInfo in the xxxBuilder::files() method.

It will improve DX by sending \SplFileInfo returned by a processor (see #90) to the MergeBuilder for example:

$fileProcessor = new FileProcessor($this->filesystem, $this->directory);

// Generate parts
$part1 = $this->pdf->html()
    ->content('pdf/invoice/part1.html.twig')
    ->processor($fileProcessor)
    ->generate()
    ->process()
;
$part2 = $this->pdf->html()
    ->content('pdf/invoice/part2.html.twig')
    ->processor($fileProcessor)
    ->generate()
    ->process()
;

// Merge
$invoice = $this->pdf->merge()
    ->fileName('invoice.pdf')
    ->files($part1, $part2) // <-- HERE
    ->processor($fileProcessor)
    ->generate()
    ->process()
;