[Microsoft.VirtualMachineImages/imageTemplates]: Lacking validation of `identity.type` field

### Resource Type

Microsoft.VirtualMachineImages/imageTemplates

### Api Version

2023-07-01

### Issue Type

Resource fails to deploy

### Other Notes

When deploying image template for Azure VM Image Builder, one can provide an user-assigned identity, which is supposed to be used by the service. In Bicep, the identity is defined like so:
```
identity: {
type: 'UserAssigned'
userAssignedIdentities: {
'${uai.id}': {}
}
}
```
However, omitting the property `type` inside the definition causes a deployment to fail because of `InvalidTemplate` error:
```
{"code": "InvalidTemplate", "message": "Deployment template parse failed: 'Required property 'type' not found in JSON. Path '', line 43, position 19.'."}
```
While this issue is quite easy to fix when deploying to a resource group, when using the very same template, but deploying as module on e.g. subscription level, makes debugging much more difficult:
```
{"code": "InvalidTemplate", "target": "/subscriptions/.../resourceGroups/.../providers/Microsoft.Resources/deployments/it", "message": "Deployment template parse failed: 'Required property 'type' not found in JSON. Path '', line 1, position 1524.'."}
```
The `position` indicated by the error is nowhere related to the real position where error occurs.

To avoid such difficult to find & fix errors, Bicep should validate if the `identity.type` field exists and mark a resource definition as invalid if the field doesn't exist.

### Bicep Repro

```
param parName string
param parLocation string = resourceGroup().location
param parIdentityName string
param parLocationSuffix string = 'we'
param parImageName string
param runSuffix string = utcNow()
param parCustomizers array

resource it 'Microsoft.VirtualMachineImages/imageTemplates@2023-07-01' = {
name: 'it-${parName}-${parLocationSuffix}'
location: parLocation
identity: {
//type: 'UserAssigned'
userAssignedIdentities: {
'${uai.id}': {}
}
}
properties: {
distribute: [
{
location: parLocation
#disable-next-line use-resource-id-functions
imageId: '/subscriptions/${subscription().subscriptionId}/resourceGroups/${resourceGroup().name}/providers/Microsoft.Compute/images/${parImageName}'
runOutputName: '${parImageName}-${runSuffix}'
type: 'ManagedImage'
}
]
source: {
type: 'PlatformImage'
publisher: 'Canonical'
offer: '0001-com-ubuntu-server-jammy'
sku: '22_04-lts-gen2'
version: 'latest'
}
customize: parCustomizers
}
}

resource uai 'Microsoft.ManagedIdentity/userAssignedIdentities@2023-01-31' = {
name: 'uai-${parIdentityName}-${parLocationSuffix}'
location: parLocation
}
```

### Bicep params
```
using 'image-template.bicep'

param parName = 'somename'
param parLocation = 'westeurope'
param parIdentityName = 'it'
param parImageName = 'imagename'
param parCustomizers = [
{
type: 'Shell'
name: 'InstallUpgrades'
inline: [
'sudo apt install unattended-upgrades'
]
}
]

```

### Confirm

- [X] I have read the troubleshooting guide and looked for duplicates.

Start from the Microsoft.VirtualMachineImages/imageTemplates type definition in this repository and reproduce the supplied Bicep example with identity.type omitted, including subscription-level module deployment. Trace whether the resource schema represents identity.type as required; done means the invalid resource definition is diagnosed and flagged before deployment, with coverage for the reported case.

Written by the indexing model from the issue text.