Adding support for non-backed enums

[ollieread]

This PR adds support for non-backed enums when casting eloquent attributes. It works based on the fact that enum cases are glorified constants, so the constant() method will work for retrieving.

It also updates the $value param type for Illuminate\Database\Eloquent\Concerns\HasAttributes::setEnumCastableAttribute from \BackedEnum to \UnitEnum|string|int. It's worth noting that the original version should have technically been \BackedEnum|string|int because there are code paths for handling when it isn't an instance.

[foremtehan]

please add tests if possible, what happen if someone modify your code later

[johanmolen]

Should the non-backed enums also be implemented in the addCastAttributesToArray method?

[ollieread]

Should the non-backed enums also be implemented in the addCastAttributesToArray method?

This is a very good point!

[ollieread]

@laravel there's a valid point raised by @johanmolen, will fix this ASAP

[johanmolen]

@ollieread awesome, if you need help let me know.

[johanmolen]

@ollieread my quick fix for now (line 310):

if ($this->isEnumCastable($key) && (! ($attributes[$key] ?? null) instanceof Arrayable)) {
    if (! isset($attributes[$key])) {
        $attributes[$key] = null;
    } else if ($attributes[$key] instanceof BackedEnum) {
        $attributes[$key] = $attributes[$key]->value;
    } else {
        $attributes[$key] = $attributes[$key]->name;
    }
}

[ollieread]

Added a fix in #43842 @johanmolen

[johanmolen]

@ollieread Awesome 💯