[BUG] `credentials.managedIdentity` missing on `Microsoft.ApiManagement/service/backends` specs

[remyblok]

API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/apimanagement/resource-manager/Microsoft.ApiManagement/stable/2024-05-01/definitions.json

API Spec version

2024-05-01

Describe the bug

In the Azure Portal it is possible to create a Backend in API Management where the backedn credentials are using Managed Identity. When deploying the backend from ARM/Bicep the managedIdentity property is missing from the specs

Expected behavior

managedIdentity is a property of the credentials for an APIM backend.

Actual behavior

managedIdentity is unrecognized as a property of the credentials for an APIM Backend. Deploying the ARM with the unrecognized property results in a succesful deployment with the correct settings.

Reproduction Steps

{
    "type": "Microsoft.ApiManagement/service/backends",
    "apiVersion": "2024-05-01",
    "name": "[format('{0}/{1}', parameters('apimServiceName'), 'my-logic-app')]",
    "properties": {
        "title": "my-logic-app",
        "resourceId": "[uri(environment().resourceManager, resourceId('Microsoft.Web/sites', 'my-logic-app'))]",
        "url": "[format('https://{0}/api', reference(resourceId('Microsoft.Web/sites', 'my-logic-app'), '2024-04-01').defaultHostName)]",
        "protocol": "http",
        "credentials": {
            // this property is working, but is not recognized in ARM/Bicep
            "managedIdentity": {
                "clientId": "[reference(resourceId('Microsoft.ManagedIdentity/userAssignedIdentities', 'uami-apim'), '2018-11-30').clientId]",
                "resource": "[parameters('apiBackedResourceUri')]"
            }
        },
        "tls": {
            "validateCertificateChain": true,
            "validateCertificateName": true
        }
    }
}

Environment

No response