[12.x] Allow sub-array notation in arrays validation

[fgaroby]

This PR allows to use the sub-array notation, in addition to the dot notation.
For example, instead of this:

class NestedArrayRequest extends FormRequest
{
    public function rules()
    {
        return [
            'nested.*.bar' => 'required',
            'nested.*.baz' => 'required'
        ];
    }
}

We can now do this:

class SubArrayRequest extends FormRequest
{
    public function rules()
    {
        return [
            'nested' => [
                '*' => [
                    'bar' => 'required',
                    'baz' => 'required',
                ],
            ],
        ];
    }
}

Usefull when the sub-array has many sub-fields.

Of course, the nested notation still works.

If this PR is OK, I also created a PR to improve the Laravel documentation.

[shaedrich]

I already attempted something similar a while ago in #55491, so don't get your hopes up. That being said, it'd be nice to have this in Laravel, so 🤞🏻