[12.x] Add support for nested array notation within loadMissing

[angus-mcritchie]

This pull request attempts to solve the same issue as this PR #54554 but was closed after @taylorotwell , the legend himself, said in this comment #54554 (comment)

Could we not re-use the logic we use for this elsewhere? It feels odd to have to recreate this logic here.

So this PR attempts to do just that.

As a refresher, this PR aims to make loadMissing consistent with load and with Eloquent methods, as I'm sure I'm not the only developer who has tried the following code and found out the hard way it doesn't work as expected.

$posts->loadMissing([
    'comments' => [
        'user' => fn($query) => $query->select(['id', 'name']),
        'relatedComments'
    ],
    'relatedPosts'
]);

Credit to @lioneaglesolutions for some of the tests 👍