PhpWeasyPrint vulnerable to SSRF and local file disclosure via the attachment option

PhpWeasyPrint is a PHP library allowing PDF generation from a URL or an HTML page. Prior to version 2.6.0, `pontedilana/php-weasyprint` fetches the content of option values server-side via `file_get_contents()` when the value looks like a URL, without restricting the URL scheme. The `attachment` option of `Pdf` is the reachable sink: any value that passes `isOptionUrl()` (`filter_var(..., FILTER_VALIDATE_URL)`) is downloaded by the PHP process and embedded into the generated PDF. Because `FILTER_VALIDATE_URL` accepts `http`, `https`, `ftp`, `file` and PHP stream wrappers such as `php://`, an attacker who can influence the `attachment` value reaches both a **Server-Side Request Forgery** primitive (e.g. internal HTTP endpoints, cloud metadata) and a local file disclosure primitive (`file://`, `php://filter/...`), with the fetched bytes exfiltrated as a PDF attachment. This is the same class of issue KnpLabs/snappy patched for its `xsl-style-sheet` option in GHSA-c5fp-p67m-gq56. The library is documented as a one-to-one substitute for KnpLabs/snappy and shares the same code shape. PhpWeasyPrint version 2.6.0 contains a patch for the issue.