Enable casting nested values

[nandi95]

Model may include arrays as a property on the model.
Would it be possible to define casts for keys on these sub keys? for example

protected $casts = [
    'settings.key' => MyEnum::class
];

Currently the logic is the following:

framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

Lines 286 to 288 in ea5172e

	$attributes[$key] = $this->castAttribute(
	$key, $attributes[$key]
	);

I imagine this could be achieved by using the data_set() helper method