Deploy Custom image with New-AzLabServicesLab

### Description

When I want to deploy a custom image I get error messages.
If I roll out an image from the store it works fine.

An error message appears that my image cannot be found. This is right. Because the MarketPlace is being looked at.

Custom:
#Compute
/subscriptions/SubscriptionID/resourceGroups/VirtualLabs/providers/Microsoft.Compute/galleries/acg_Private_LabServices/images/W11-Base

Marketplace:
/subscriptions/SubscriptionID/resourceGroups/VirtualLabs/providers/Microsoft.LabServices/labPlans/Private-LabServices

### Issue script & Debug output

```PowerShell
Az.LabServices.internal\New-AzLabServicesLab : Resource with ID '/subscriptions/Private/resourceGroups/VirtualLabs/providers/Microsoft.LabServices/labPlans/Private-LabServices/image
s/AzureLabServices.General.W11-Base' cannot be found or does not exist.
```

### Environment data

```PowerShell
5.1.22000.1335
```

### Module versions

```PowerShell
Script 0.1.0 Az.LabServices
```

### Error output

```PowerShell
Az.LabServices.internal\New-AzLabServicesLab : Resource with ID '/subscriptions/SubscriptionID/resourceGroups/VirtualLabs/providers/Microsoft.LabServices/labPlans/CT-LabServices/image
s/AzureLabServices.General.W11-Base' cannot be found or does not exist.
```

Start at the New-AzLabServicesLab entry point and reproduce the deployment with the custom Azure Compute Gallery image path from the issue, comparing it with the working Marketplace image case. Done means the custom image is resolved from the supplied path instead of the Marketplace lookup and the lab deploys without the resource-not-found error.

Written by the indexing model from the issue text.