[9.x] Add ability to define "with" relations as a nested array

[timacdonald]

Purpose

This PR aims to add the ability to specify relationship "withs" as a PHP array, rather than dot notation.

Usage

// before...

User::with([
    'avatar',
    'posts.tags',
    'posts.author',
    'posts.featureImage',
    'posts.comments.tags' => fn ($q) => $q->latest(),
])->get();

// after...

User::with([
    'avatar',
    'posts' => [
        'tags',
        'author',
        'featureImage',
        'comments' => [
            'tags' => fn ($q) => $q->latest(),
        ],
    ],
])->get();

Advanced

// supports chaining...

User::with([
    'posts' => fn ($q) => $q->withCount('comments')->with([
        'comments' => [
            'tags',
        ],
    ]),
])->get();

// supports specifying attributes inline, although I think with this notation you
// should probably be specifying selects via a constraint.

User::with([
    'posts:id,title,user_id' => [
        'comments:id,content,post_id' => [
            'tags',
        ],
    ],
])->get();

// supports mixing notation. Not saying you should, just saying you can 🙈

User::with([
    'posts' => [
        'comments',
    ],
    'posts.image',
])->get();

// merges constraints from different notation - again, not saying you should. just saying you can.

User::with([
    'posts.comments' => fn ($q) => $q->with('tags'),
    'posts:id,title,user_id' => [
        'comments' => fn ($q) => $q->withCount('tags'),
    ],
])->get();

What isn't supported

I should note that merging of selects from different notation is not supported. I don't see this as a problem, as the current dot notation suffers from the same constraint...

// does not select "id" and "name"
User::query()
    ->with(['users:id,name'])
    ->with(['users:email'])
    ->get();

// nor does this...

User::query()
    ->with(['users' => fn ($q) => $q->select(['id', 'name'])])
    ->with(['users:email'])
    ->get();

and so the following is explicitly not supported...

User::with([
    'posts.comments' => fn ($q) => $q->addSelect(['id']),
    'posts' => [
        'comments' => fn ($q) => $q->addSelect(['post_id'])
    ],
])->get();

User::with([
    'posts' => [
        'comments:post_id,title'
    ],
    'posts.comments:id',
])->get();

or any other mixture of notation with duplicate selects.

Notes

I'd love to get some eyes on this from a range of people who are deep in Eloquent land.

[OzanKurt]

Does this also work on ->load()?