[12.x] Add BackedEnum support for session keys

[ahinkle]

This PR adds enum support for Session keys. Laravel already handles BackedEnums gracefully in Gate, Queue, Cache, RateLimiter, and elsewhere - this pattern follows the same.

If you're managing a multi-step checkout flow across several components, you probably have something like:

enum CheckoutSession: string
{
    case Cart = 'checkout.cart';
    case ShippingAddress = 'checkout.shipping_address';
    case PaymentMethod = 'checkout.payment_method';
}

Right now you have to do ->value everywhere:

session()->put(CheckoutSession::Cart->value, $items);
session()->get(CheckoutSession::Cart->value);

This PR lets you drop the ->value:

session()->put(CheckoutSession::Cart, $items);
session()->get(CheckoutSession::Cart);

Uses enum_value() to normalize keys, same pattern as the rest of the framework.