[HttpKernel] Add a controller argument resolver for backed enums

[ogizanagi]

Fixes #16357

Adding a ref to the BackedEnumValueResolver introduced in 6.1 to https://symfony.com/doc/current/controller/argument_value_resolver.html

Added before RequestAttributeValueResolver since it has a higher priority.

Don't know however where we could showcase such an example:

Given:

namespace App\Model;

enum Suit: string 
{
    case Hearts = 'H';
    case Diamonds = 'D';
    case Clubs = 'C';
    case Spades = 'S';
}

and the controller:

class CardController
{
    #[Route('/cards/{suit}')]
    public function list(Suit $suit): Response
    {
        // [...]
    }
}

A request to /cards/H would inject the Suit::Hearts enum case into the controller $suit argument.

[javiereguiluz]

Maxime, thanks a lot for contributing this doc update. I agree with you that it's difficult to understand this feature without an example (maybe it will be easier when most PHP developers are used to Enums). So, in 4fe62c6 I've added the code example that you proposed.

[ogizanagi]

That's great, thank you Javier. 👍🏻
For some reasons, the issue #16357 wasn't closed during merge?