feat: add lazy template rendering to stream HTML at send time

[Jean-Beru]

Q	A
Gotenberg API version ?	8.x
Bug fix ?	no
New feature ?	yes
BC break ?	no

Description

When calling content($template, $context) (or header() / footer()), the Twig template is rendered immediately and the resulting HTML is kept in memory until the request is sent to Gotenberg.

This PR adds a $lazy argument to content(), header() and footer(). When set to true, the template is only rendered while the HTTP request body is sent. Peak memory usage is then a single Twig output chunk instead of the full document.

Lazy rendering is disabled by default to keep BC promise.

Example

public function __invoke(GotenbergPdfInterface $gotenberg): Response
{
    return $gotenberg->html()
        ->content('invoice.html.twig', ['invoice' => $invoice], lazy: true)
        ->generate()
        ->stream();
}

Screenshots

Example: 5000 lines containing an inlined svg + a paragraph.

Before

After