[7.x] Scoped resource routes

[iamgergo]

This PR adds the ability to make resource routes scoped.

For a while, we have the ability to scope routes when using Implicit Route Model Binding. This is very useful because it provides a quick and easy route key customization and respects the parent-child relations as well:

Route::get('api/users/{user}/posts/{post:slug}', function (User $user, Post $post) {
    return $post;
});

This PR implements the same, but for resource routes, so if we wish to use the same functionality for a resource URL structure, we don't need to define the routes one-by-one to use this feature or writing a custom route binding resolution logic for each model.

---

Usage:

// Using the default route keys
Route::resource('users.tasks', TasksController::class)->scoped();

// Customizing some route keys (only the task gets replaced)
Route::resource('users.tasks', TasksController::class)->scoped([
    'task' => 'slug',
]);

If we type hint our models in the controller methods using implicit route model binding, it will behave the same as the closure-based route above.

TasksController extends Controller
{
    // ...

    public function show(User $user, Task $task)
    {
        //
    }

   // ...
}

---

I don't think there is any breaking change so I chose the 7.x branch.

Also, if the PR is accepted, I'll open a PR for the docs as well.

[iamgergo]

I'm not sure why did the tests fail.

@driesvints If you'll have some time, could you restart the tests, please? Thanks!

[driesvints]

@iamgergo seems you got it.

Did you test this with route caching btw?

[iamgergo]

@driesvints Well, I ran the php artisan route:cache command and it was working well. Or you meant something else?

Also, in the latest commit, I fixed the null column value by getting the default one using the getRouteKeyName method.

[driesvints]

@iamgergo yep that's what I meant.