PDF attachments

[devmount]

This PR offers a trait to add file attachments to PDF files.

It can be used similar to the already existing traits:

use fpdf\PdfDocument;
use fpdf\Traits\PdfAttachmentTrait;

class AttachmentDocument extends PdfDocument
{
    use PdfAttachmentTrait;
}

$file = "test/attached.txt";

$pdf = new AttachmentDocument();
$pdf->addPage();
$document->attach($file);
$document->openAttachmentPane();

$pdf->output();

It also provides tests and documentation for this trait.

Fixes #4

[codecov-commenter]

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