[ApiLoader] Support _format resolving

[ahmed-bhs]

Q	A
Branch?	main
Bug fix?	no, yes
New feature?	no, yes
BC breaks?	no
Deprecations?	no
Tickets	#There's no ticket
License	MIT

Hello there,

Currently, it is not possible to resolve the default value for the _format parameter under a custom operation definition.
This is due to the ApiLoader which forces this value to null, no matter what value we set in the configuration.
This fix, makes it possible to take into account the definition of the defaults._format parameter and have the right value under the request route params.

Use case:
Imagine that we have a resource which is used to export both CSV and XLSX data.

resources.yaml:

resources:
    App\Entity\Activity\Contract:
        collectionOperations:
            export:
                method: 'GET'
                path: /contracts/export.{_format}
                defaults:
                    _format: "xlsx"
               requirements:
                   _format: "xlsx|csv"
                controller: App\Controller\Api\ExportContractController

ExportContractController:

final class ExportContractController extends AbstractController
{
    public function __invoke($_format, $data)
    {
    // before the fix, $_format return null
    dd($_format);
    // after the fix, $_format return 'xlsx'
    
    }
}

[alanpoulain]

Seems alright to me. Could you add a CHANGELOG entry and if possible a PR for the documentation?

[soyuka]

thanks @ahmed-bhs !