Cannot deserialize change notification - JsonException due to failure converting ChangeType

[sebastienlange]

Describe the bug

This is very similar to #998

Attempting to deserialize the payload of a notification received by a webhook fails with:
The JSON value could not be converted to System.Nullable`1[Microsoft.Graph.Models.ChangeType]. Path: $.value[0].changeType | LineNumber: 5 | BytePositionInLine: 35.

The same problem occurs for Lifecycle notifications:
JSON input formatter threw an exception: The JSON value could not be converted to System.Nullable`1[Microsoft.Graph.Models.LifecycleEventType]. Path: $.value[0].lifecycleEvent | LineNumber: 0 | BytePositionInLine: 472.

Expected behavior

The notifications variable contains a valid ChangeNotificationCollection object with one ChangeNotification.

How to reproduce

The following code will reproduce:

`// This is an actual payload sent by the service that failed in my application
string payload = @"{
""value"": [
{
""subscriptionId"": ""5c748643-b830-4b11-a165-cc4b59c9820c"",
""subscriptionExpirationDateTime"": ""2021-05-15T09:06:46.5737745-07:00"",
""changeType"": ""created"",
""resource"": ""Users/1023ccca-b008-40f6-950a-aa1951a1f0d4/Messages/AAMkADY0ZjM2ZWJlLTEwOGUtNGNiOS04MTFiLWE3Y2UwNjRiZDNiMQBGAAAAAACWRvY6Vjb0SabVPshsbtVRBwAtISQkTseOQLhQ9a0koD2UAAAAAAEMAAAtISQkTseOQLhQ9a0koD2UAAB9Mb9SAAA="",
""resourceData"": {
""@odata.type"": ""#Microsoft.Graph.Message"",
""@odata.id"": ""Users/1023ccca-b008-40f6-950a-aa1951a1f0d4/Messages/AAMkADY0ZjM2ZWJlLTEwOGUtNGNiOS04MTFiLWE3Y2UwNjRiZDNiMQBGAAAAAACWRvY6Vjb0SabVPshsbtVRBwAtISQkTseOQLhQ9a0koD2UAAAAAAEMAAAtISQkTseOQLhQ9a0koD2UAAB9Mb9SAAA="",
""@odata.etag"": ""W/""CQAAABYAAAAtISQkTseOQLhQ9a0koD2UAAB9GwlV"""",
""id"": ""AAMkADY0ZjM2ZWJlLTEwOGUtNGNiOS04MTFiLWE3Y2UwNjRiZDNiMQBGAAAAAACWRvY6Vjb0SabVPshsbtVRBwAtISQkTseOQLhQ9a0koD2UAAAAAAEMAAAtISQkTseOQLhQ9a0koD2UAAB9Mb9SAAA=""
},
""clientState"": ""GraphTutorialState"",
""tenantId"": ""dfa81919-e5d3-4357-82ae-6b1e672fde09""
}
]
}";

var jsonOptions = new JsonSerializerOptions
{
PropertyNamingPolicy = JsonNamingPolicy.CamelCase
};

// Deserialize the JSON payload into a list of ChangeNotification
// objects
try
{
var notifications = JsonSerializer.Deserialize(payload, jsonOptions);
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
}`

SDK Version

5.68.0

Latest version known to work for scenario above?

No response

Known Workarounds

Creating my own ChangeNotificationCollection class with own ChangeNotification class where ChangeType and LifecycleEvent are string is working.

Debug output

Click to expand log

```

</details>


### Configuration

- OS: Windows 11
- architecture: x64
- It's not specific to that configuration

### Other information

Of course, the following code also reproduces the problem:

`var changeType = JsonSerializer.Deserialize<Microsoft.Graph.Models.ChangeType>("created");`

[andrueastman]

Thanks for raising this @sebastienlange

Any chance you are able to replicate this using the serialization helpers instead? https://learn.microsoft.com/en-us/openapi/kiota/serialization?tabs=csharp#serialization-helpers