[8.x] Chainable casts

[hotmeteor]

This PR adds new functionality for "chainable casts", allowing for more reusable and flexible casting of attributes. Chainable casts are defined by using an array of cast types and/or custom cast classes.

For example, an attribute could first be cast as json, and then encrypted, all without needing to add new cast rules or create multiple custom casts. Chainable casts can contain as many casts as necessary, including custom cast classes.

protected $casts = [
    'birth_date' => 'date',
    'password' => [
        'string',
        'encrypted',
    ],
    'number' => [
        'integer',
        'encrypted',
    ],
    'options' => [
        'array',
        'encrypted',
    ],
    'hash' => [
        ReverseCaster::class,
        SlugifyCaster::class,
        Base64Caster::class,
    ],
];

[taylorotwell]

I think I'm going to hold off on this for now. I talked about this with Jason last week and came to the conclusion that I couldn't think of any uses for this outside of encryption, and it introduces a new concept that we would have to maintain / improve / change going forward. Chainable custom casts aren't really necessary IMO because the custom cast already can just implement its own set of options within itself. Encryptable primitives like encrypted integers or booleans don't make much sense either.

[ryangjchandler]

I think this problem could be solved in userland with a custom caster and slightly different syntax too:

protected $casts = [
    'password' => ChainedCast::class . ':string,encrypted',
];