[10.x] Add support for BackedEnum in Collection groupBy method

[osbre]

I tried to group the Eloquent model collection by the Enum-casted key and got this error:

TypeError
array_key_exists(): Argument #1 ($key) must be a valid array offset type

This PR aims to fix that by adding support for BackedEnum in groupBy method.

Code example

$attributesByCategory = Attribute::whereIn('category', [
      AttributeCategory::LOREM,
      AttributeCategory::IPSUM,
  ])
  ->get()
  ->groupBy('category');

enum AttributeCategory: string
{
    case LOREM  = 'lorem';
    case IPSUM   = 'ipsum';
}

class Attribute extends Model
{
    protected $casts = [
        'category' => AttributeCategory::class,
    ];
}