Add support for custom scoped route model bindings

[bakerkretzmar]

This PR adds support for custom scopes in route model bindings.

Laravel 7 added the ability to scope nested route model bindings on a specific attribute:

// This route will look for a Post using the 'slug' attribute
Route::get('users/{user}/posts/{post:slug}', function (User $user, Post $post) {
    return $post;
})->name('users.posts');

Ziggy already supports passing an object parameter and automatically detecting an id key if one is present:

route('users.posts', [{ id: 4, name: 'Jacob' }, 'ziggy-v1-announcement'])
// Returns http://example.com/users/4/posts/ziggy-v1-announcement

This PR adds support for passing an object as a parameter with a custom binding scope:

route('users.posts', [{ id: 4, name: 'Jacob' }, { published: false, slug: 'ziggy-v1-announcement' }])
// Now also returns http://example.com/users/4/posts/ziggy-v1-announcement

Similarly to the existing behaviour with id, this only works if there are no conflicting route parameter names, so the example above won't necessarily work if there is another parameter explicitly called just slug.

[jakebathman]

Good stuff, LGTM 👍