[10.x] Add ability to set a custom class for the AsCollection and AsEncryptedCollection casts

[osbre]

Example:

protected $casts = [
    'elements' => AsCollection::class.':'.ElementCollection::class,
    // or
    'elements' => AsEncryptedCollection::class.':'.ElementCollection::class,
];

(In a similar way to AsEnumCollection)

protected $casts = [
    'statuses' => AsEnumCollection::class.':'.ServerStatus::class,
];