[4.x] use method instead of array for casts on PersonalAccessToken

[cosmastech]

I need to be able to add some additional columns to my personal_access_tokens table. This makes it possible that I can do the following:

class TeamworksToken extends PersonalAccessToken
{
    protected function casts()
    {
        return parent::casts() + ['is_impersonation' => 'bool'];
    }
}

[cosmastech]

Taking a look at how https://github.com/laravel/framework/blob/a296e813d041ddff000d856aa2f7ae1cd4d616b7/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php#L196 is setup, for my use case, it wouldn't matter, since a casts() method on the child class would get merged in.

Possibly other people would want to manipulate the casts in a different way. 🤷

If you don't want to accept this PR because it's fine as is, I'll make a follow up to just add in the proper typehints on the model.

[taylorotwell]

Since it's fine as is we can just do the type-hints PR.