mgmt: Add async_approval_notification_channels property to client model

[trivm963]

Changes

If a client is set up to use a CIBA (Client Initiated Backchannel Authentication) grant type, it is necessary to configure a notification channel for it. On the console it looks like this:

As per the docs, Management API itself has async_approval_notification_channels as a body parameter for requests to create or update clients at the /v2/clients endpoint.

However the async_approval_notification_channels property is missing from the client model in the Auth0 SDK, so when there is an attempt to create a client with CIBA grant type it fails with the following error:

Auth0.Core.Exceptions.ErrorApiException: Application must have at least one channel configured in 'async_approval_notification_channels' when urn:openid:params:grant-type:ciba is enabled

This PR adds the necessary async_approval_notification_channels property to the client model.

Testing

An end-to-end test was carried out to check that the SDK change works and creates a client as intended. This involved creating via Management API an M2M client with Client Credentials and CIBA grant types, with a working Guardian Push notification channel.

There are not additional automated tests added because the change is limited to adding a property to an existing model class and similar model properties in the codebase don't have dedicated unit tests.

• This change adds unit test coverage

• This change adds integration test coverage

• This change has been tested on the latest version of the platform/language

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors