Serialisation/deserialisation no longer works

Describe the bug

Native serialisation and deserialisation of models no longer works in v2.

In the msgraph-sdk-php-core readme.md file it states:

We provide Microsoft Graph models for easy serialization and deserialization.

If you would like to leverage the models we provide, please take a look at the [Microsoft Graph PHP SDK](https://packagist.org/packages/microsoft/microsoft-graph) and for beta models - the [Microsoft Graph Beta PHP SDK](https://packagist.org/packages/microsoft/microsoft-graph-beta).

Sadly this is no longer the case. As soon as any nested object is added to a model, serialisation fails as the embedded closures cannot be serialised.

Admittedly this text is obviously written for V1 as it references Graph Beta, but I don't think it has been intentional to remove the ability to natively serialise models. If that is the case, I think that is a bad move. I don't think there in anything in the upgrade.md file that suggests you can no longer natively serialize and, importantly, deserialize models. No examples exist describing how to do this using the SDK.

Take for example the Message model, in it's basic form you can serialise it, however as soon as you add an ItemBody element you can't because the backing store now contains a subscriber for changes to the ItemBody, which is implemented as a closure. This prevents the Message model from being serialised using native PHP serialize/unserialize methods.

My usage case is that I need to offload the sending of messages to a background task. The messages are created when forms are submitted etc. and saved to a scheduler list which picks them up via a command line background task, and then performs the actual sending of the Message object.

There are already issues that hint towards this problem but not to the actual cause of the error:
https://github.com/microsoftgraph/msgraph-sdk-php/issues/1471
https://github.com/microsoftgraph/msgraph-sdk-php/issues/1470
https://github.com/microsoftgraph/msgraph-sdk-php/issues/1452

Expected behavior

Native serialize and unserialize methods should work with all SDK models.

How to reproduce

This simple code will crash when attempting to serialise the Message object:

    $message = new Message();
    $message->setSubject('Test');
    $body = new ItemBody();
    $body->setContentType(new BodyType(BodyType::HTML));
    $body->setContent('<p>Test</p>');
    $message->setBody($body); // Causes a subscription to be set on ItemBody
    $test = serialize($message); // This would be stored for the background task.

SDK Version

v2.12.0

Latest version known to work for scenario above?

v1

Known Workarounds

None.

Debug output

Click to expand log

object(Exception)#705 (7) { ["message":protected]=> string(41) "Serialization of 'Closure' is not allowed" ["string":"Exception":private]=> string(0) "" ["code":protected]=> int(0) ["file":protected]=> string(64) "redacted.php" ["line":protected]=> int(220) ["trace":"Exception":private]=> array(5) { ... } ```

Configuration

No response

Other information

No response

Start with the Message and ItemBody models and the backing-store subscription described in the report, then reproduce the failure with PHP serialize() and unserialize(). Review readme.md and upgrade.md for the serialization claims. Done means native serialization and deserialization work for SDK models containing nested objects, or the documentation clearly states the supported alternative.