Adds custom unattend XML file selection paths

Adds parameters for x64 and arm64 unattend XML file paths.
Updates the BuildFFUVM_UI to let the user pick specific XML files to copy or inject instead of requiring them to exist in the `unattend` folder.
Adds validation to ensure the selected unattend XML files are not empty.
Updates documentation to reflect the new functionality.

FFUDevelopment/FFUUI.Core/FFUUI.Core.psm1

@@ -112,6 +112,8 @@ function Get-GeneralDefaults {
     $userAppListPath = Join-Path -Path $appsPath -ChildPath "UserAppList.json"
     $driversJsonPath = Join-Path -Path $driversPath -ChildPath "Drivers.json"
     $deviceNamePrefixesPath = Join-Path -Path $unattendPath -ChildPath "prefixes.txt"
+    $unattendX64FilePath = Join-Path -Path $unattendPath -ChildPath "unattend_x64.xml"
+    $unattendArm64FilePath = Join-Path -Path $unattendPath -ChildPath "unattend_arm64.xml"
 
     return [PSCustomObject]@{
         # Build Tab Defaults
@@ -134,6 +136,8 @@ function Get-GeneralDefaults {
         CopyUnattend                   = $false
         CopyPPKG                       = $false
         InjectUnattend                 = $false
+        UnattendX64FilePath            = $unattendX64FilePath
+        UnattendArm64FilePath          = $unattendArm64FilePath
         DeviceNamingMode               = 'None'
         DeviceNameTemplate             = ''
         DeviceNamePrefixesPath         = $deviceNamePrefixesPath