[BUG] `Microsoft.DevTestLab/schedules` API does not return `notificationSettings.webhookUrl` property in response

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/devtestlabs/resource-manager/Microsoft.DevTestLab/DevTestLabs/stable/2018-09-15/DTL.json

### API Spec version

2018-09-15

### Describe the bug

`Microsoft.DevTestLab/schedules` API for [schedules](https://learn.microsoft.com/en-us/rest/api/dtl/schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP) and [global schedules](https://learn.microsoft.com/en-us/rest/api/dtl/global-schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP) does not return [`notificationSettings.webhookUrl` property](https://learn.microsoft.com/en-us/rest/api/dtl/global-schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP#notificationsettings) in response even if the property is set in request. This happens too when [creating auto-shutdown schedule](https://learn.microsoft.com/en-us/azure/virtual-machines/auto-shutdown-vm?tabs=portal#configure-auto-shutdown-for-a-virtual-machine) with webhook URL in virtual machine using portal, where webhook URL is empty after saving and refreshing.

### Expected behavior

After sending request to create auto-shutdown schedule, whether through [REST API](https://learn.microsoft.com/en-us/rest/api/dtl/global-schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP) with `notificationSettings.webhookUrl` or portal with "webhook URL" specified (for example `https://www.bing.com/2/4`), it should be able to verify the setting of webhook URL through the response below.

API response
```json
...
"notificationSettings": {
"emailRecipient": "REDACTED",
"status": "Enabled",
"timeInMinutes": 15,
"webhookUrl": "https://www.bing.com/2/4"
},
...
```

Portal response

### Actual behavior

The actual response is that `notificationSettings.webhookUrl` is missing from API response and portal "webhook URL" is empty after saving and refreshing.

API response
```json
...
"notificationSettings": {
"emailRecipient": "REDACTED",
"status": "Enabled",
"timeInMinutes": 15
},
...
```

Portal response

### Reproduction Steps

To reproduce the issue with REST API for [schedules](https://learn.microsoft.com/en-us/rest/api/dtl/schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP), use the following request and body with existing DevTest labs.

Request
```
PUT https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.DevTestLab/labs/{labName}/schedules/{name}?api-version=2018-09-15
```

Body
```json
{
"location": "japaneast",
"properties": {
"dailyRecurrence": {
"time": "0900"
},
"notificationSettings": {
"status": "Enabled",
"timeInMinutes": 30,
"webhookUrl": "https://www.bing.com/2/4"
},
"status": "Enabled",
"taskType": "LabVmsShutdownTask",
"timeZoneId": "India Standard Time"
},
"tags": {
"environment": "Production"
}
}
```

To reproduce the issue with REST API for [global schedules](https://learn.microsoft.com/en-us/rest/api/dtl/global-schedules/create-or-update?view=rest-dtl-2018-09-15&tabs=HTTP), use the following request and body with existing virtual machine.

Request
```
PUT https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.DevTestLab/schedules/{name}?api-version=2018-09-15
```

Body
```json
{
"location": "japaneast",
"properties": {
"dailyRecurrence": {
"time": "1100"
},
"notificationSettings": {
"emailRecipient": "REDACTED",
"status": "Enabled",
"timeInMinutes": 15,
"webhookUrl": "https://www.bing.com/2/4"
},
"status": "Disabled",
"targetResourceId": "/subscriptions/REDACTED/resourceGroups/REDACTED/providers/Microsoft.Compute/virtualMachines/REDACTED",
"taskType": "ComputeVmShutdownTask",
"timeZoneId": "Central Standard Time"
},
"tags": {
"Environment": "Production"
}
}
```

To reproduce the issue with portal, follow the steps in [documentation](https://learn.microsoft.com/en-us/azure/virtual-machines/auto-shutdown-vm?tabs=portal#configure-auto-shutdown-for-a-virtual-machine) and set "webhook URL". The behavior for DevTest labs in portal is not tested but considering that its REST API is problematic, the issue should happen for DevTest labs in portal too.

### Environment

REST API request is sent with [`terraform-provider-azurerm`](https://github.com/hashicorp/terraform-provider-azurerm) `main` branch.

Start with specification/devtestlabs/resource-manager/Microsoft.DevTestLab/DevTestLabs/stable/2018-09-15/DTL.json and compare the schedules and global schedules definitions with the reproduction requests. Verify whether webhookUrl is represented in the schema and reproduce the REST response; done means the documented API response preserves notificationSettings.webhookUrl for both schedule types.

Written by the indexing model from the issue text.