Add function to create a new nuget.config file (#8170)

Add a function to create a NuGet.config file to consume packages  from the specified feed URL using the specified credentials.  

  - Also, exclude the `nuget.config` file under `src/Modules` from being clean by `Start-PSBuild -clean`

build.psm1

@@ -271,7 +271,8 @@ function Start-PSBuild {
         Push-Location $PSScriptRoot
         try {
             # Excluded sqlite3 folder is due to this Roslyn issue: https://github.com/dotnet/roslyn/issues/23060
-            git clean -fdx --exclude .vs/PowerShell/v15/Server/sqlite3
+            # Excluded src/Modules/nuget.config as this is required for release build.
+            git clean -fdx --exclude .vs/PowerShell/v15/Server/sqlite3 --exclude src/Modules/nuget.config
         } finally {
             Pop-Location
         }
@@ -3043,3 +3044,35 @@ function New-TestPackage
 
     [System.IO.Compression.ZipFile]::CreateFromDirectory($packageRoot, $packagePath)
 }
+
+function New-NugetConfigFile
+{
+    param(
+        [Parameter(Mandatory=$true)] [string] $NugetFeedUrl,
+        [Parameter(Mandatory=$true)] [string] $FeedName,
+        [Parameter(Mandatory=$true)] [string] $UserName,
+        [Parameter(Mandatory=$true)] [string] $ClearTextPAT,
+        [Parameter(Mandatory=$true)] [string] $Destination
+    )
+
+    $nugetConfigTemplate = @'
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="[FEEDNAME]" value="[FEED]" />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+  <packageSourceCredentials>
+    <[FEEDNAME]>
+      <add key="Username" value="[USERNAME]" />
+      <add key="ClearTextPassword" value="[PASSWORD]" />
+    </[FEEDNAME]>
+  </packageSourceCredentials>
+</configuration>
+'@
+
+    $content = $nugetConfigTemplate.Replace('[FEED]', $NugetFeedUrl).Replace('[FEEDNAME]', $FeedName).Replace('[USERNAME]', $UserName).Replace('[PASSWORD]', $ClearTextPAT)
+
+    Set-Content -Path (Join-Path $Destination 'nuget.config') -Value $content -Force
+}