Added loadDeferredProps() method to AssertableInertia class

[pascalbaljet]

This PR introduces a loadDeferredProps() method on the AssertableInertia class, similar to testing partial reloads (see PR #738).

Inertia::render('Index', [
    'foo' => 'bar',
    'deferred1' => Inertia::defer(fn () => 'baz'),
    'deferred2' => Inertia::defer(fn () => 'qux', 'custom'),
    'deferred3' => Inertia::defer(fn () => 'quux', 'custom'),
]);

//

$response->assertInertia(function (AssertableInertia $inertia) {
    $inertia->where('foo', 'bar');
    $inertia->missing('deferred1');
    $inertia->missing('deferred2');
    $inertia->missing('deferred3');

    $inertia->loadDeferredProps(function (AssertableInertia $inertia) {
        $inertia->where('deferred1', 'baz');
        $inertia->where('deferred2', 'qux');
        $inertia->where('deferred3', 'quux');
    });
});

You can also load a specific group.

$inertia->loadDeferredProps('default', function (AssertableInertia $inertia) {
    //
});

Or multiple.

$inertia->loadDeferredProps(['default', 'custom'], function (AssertableInertia $inertia) {
    //
});