Private endpoint sub-object is missing

### Description

'Get-AzEventGridDomain' does not return any information about private endpoint.

Testing with az CLI returns the whole package "privateEndpointConnections".

### Issue script & Debug output

```PowerShell
N/A
```

### Environment data

```PowerShell
N/A
```

### Module versions

```PowerShell
Az.EventGrid 1.6.0
```

### Error output

```PowerShell
N/A
```

Start with the Get-AzEventGridDomain cmdlet and compare its returned object with the Azure CLI output described in the issue. Verify the Event Grid domain response includes the privateEndpointConnections information, then add coverage for that returned sub-object if the repository provides a relevant test location.

Written by the indexing model from the issue text.