[2.x] Introduce ProvidesInertiaProps interface

[pascalbaljet]

This PR is an addition to #746 and inspired by this comment. Given the one-letter difference in interface names, we might have to do some renaming to prevent confusion.

While Inertia::render() already support Arrayable as props, there's still room left to improve how we organize and reuse props, especially for complex pages. This new ProvidesInertiaProps interface has one method that allows you to return multiple props from a class, and then use multiple of these classes to gather your props for a page.

Here's an example of a class implementing this interface:

class LocalizationProps implements ProvidesInertiaProps
{
    public function toInertiaProps(RenderContext $context): iterable
    {
        return [
            'currentLocale' => app()->getLocale(),
            'fallbackLocale' => config('app.fallback_locale', 'en'),
            'availableLocales' => array_keys(config('app.locales', [])),
        ];
    }
}

You can use this directly in the render() and with() methods:

public function index(LocalizationProps $localizationProps)
{
    return Inertia::render('UserProfile', $localizationProps);

    // or...

    return Inertia::render('UserProfile')->with($localizationProps);
}

Even cooler, you can combine these prop classes with your other props in an array (without using the spread operator):

Inertia::render('UserProfile', [
    'user' => auth()->user(),
    $localizationProps,
]);

For complex pages with many reusable props, this can be a nice cleanup:

public function index(LocalizationProps $localizationProps, PermissionsProps $permissionProps)
{
    return Inertia::render('UserProfile', [
        'user' => auth()->user(),
        $localizationProps,
        $permissionProps,
    ]);

    // or...

    return Inertia::render('UserProfile')
        ->with('user', auth()->user())
        ->with($localizationProps)
        ->with($permissionProps);
}

The RenderContext object contains the Request instance and the component name.

[danmatthews]

Love this @pascalbaljet - using it with ->with will be how I use it i'd think, I think it's a nice backwards compatible QOL improvement along with the other PR.