[13.x] Support Guzzle 8 and persistent connections across php-fpm requests

[GrahamCampbell]

This PR adds support for Guzzle 8, while maintaining Guzzle 7 support. Guzzle 8 comes with a significant new feature: PHP 8.5+ users with ext-curl installed can have connections shared across php-fpm requests. I've implemented this with the following DX:

// app/Providers/AppServiceProvider.php
use Illuminate\Support\Facades\Http;
use Illuminate\Http\Client\PersistentTransport;

public function boot(): void
{
    Http::globalPersistentTransport(PersistentTransport::Preferred);
}

The PersistentTransport surfaces 3 options:

• None: Do not try and share connections. This is the current behaviour, and remains the default.
• Preferred: Try to share connections, but if we are not able to, silently fallback.
• Required: Try to share connections, and if we are not able to, fail loud.

[GrahamCampbell]

Header names might not be strings because PHP forces numeric strings to ints, to array aka array<array-key, mixed> really is the right type here!

[GrahamCampbell]

Rebased to get a clean CI run. Guzzle 7 gets exercised by the prefer lowest tests, and Guzzle 8 by the rest.