Add experimental VM networking opt-in for Hyper-V builds

The Hyper-V switch selection in the UI previously did not connect the build VM to the network during provisioning. Since internet-connected Sysprep and capture flows are still experimental, this introduces an explicit "Enable VM Networking" checkbox to the Hyper-V Settings page that defaults to off.

- Adds the `-EnableVMNetworking` parameter to BuildFFUVM.ps1 to conditionally attach the Hyper-V network adapter during VM creation.
- Persists the setting through FFU config files and blocks UI execution if enabled without a valid switch.
- Refactors WPF event handlers in FFUUI.Core to fix dropdown scoping errors.
- Updates documentation and the sample configuration file to reflect the new behavior.

FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1

@@ -140,6 +140,7 @@ function Get-GeneralDefaults {
         RemoveUpdates                  = $false 
         RemoveDownloadedESD            = $true
         # Hyper-V Settings Defaults
+        EnableVMNetworking             = $false
         DiskSizeGB                     = 50
         MemoryGB                       = 4
         Processors                     = 4