[12.x] Add newRequest() to Pool and Batch

[cosmastech]

This adds convenience for folks who want to create a new request without having to name it.

function addPromiseMapping($promise) {
    $promise->get('https://laravel.com/careers')
        ->then(fn ($response) => str_contains($response->body(), 'software engineer'));
}

Http::pool(function (Pool $pool) {
    addPromiseMapping($pool->newRequest());
    addPromiseMapping($pool->newRequest());
});

Prior to this, you would have to name the request (via Pool@as()) even if that wasn't convenient for the use case.