Add support for OneToMany/ManyToOne/ManyToMany relationships

[kbond]

use function Zenstruck\Foundry\create;
use function Zenstruck\Foundry\factory;

// ManyToOne
create(Post::class, [
    'category' => $category, // $category is instance of Category
]);
create(Post::class, [
    // Proxy objects are converted to object before calling Post::setCategory()
    'category' => create(Category::class, ['name' => 'My Category']),
]);
create(Post::class, [
    // Factory objects are persisted before calling Post::setCategory()
    'category' => factory(Category::class, ['name' => 'My Category']),
]);

// OneToMany
create(Category::class, [
    'posts' => [
        $post, // $post is instance of Post, Category::addPost($post) will be called during instantiation

        // Proxy objects are converted to object before calling Category::addPost()
        create(Post::class, ['title' => 'Post B', 'body' => 'body']),

        // Factory objects are persisted before calling Category::addPost()
        factory(Post::class, ['title' => 'Post A', 'body' => 'body']),
    ],
]);

// ManyToMany
create(Post::class, [
    'tags' => [
        $tag, // $tag is instance of Tag, Post::addTag($tag) will be called during instantiation

        // Proxy objects are converted to object before calling Post::addTag()
        create(Tag::class, ['name' => 'My Tag']),

        // Factory objects are persisted before calling Post::addTag()
        factory(Tag::class, ['name' => 'My Tag']),
    ],
]);

[weaverryan]

Would the model factory usage look like (this includes #5) this?

// previously use CategoryFactory to create some categories

PostFactory::create([
    'categories' => CategoryFactory::randomSet(3)
]);

And if so, this wouldn't work before this PR because it would have tried setting Proxy objects? And this PR unwraps them?

[kbond]

Would the model factory usage look like (this includes #5) this?

Correct, and this:

PostFactory::createMany(4, [
    'categories' => CategoryFactory::randomSet(3)
]);

Would create 4 Posts with the same 3 categories. You would have to wrap the attributes in a closure to get a different set of categories for each of the 4 posts:

PostFactory::createMany(4, function () {
    return [
        'categories' => CategoryFactory::randomSet(3)
    ];
});

this wouldn't work before this PR because it would have tried setting Proxy objects?

Correct, only ManyToOne worked before this PR.