Add script to download nugets from release definition

scripts/download-internal-nugets.ps1

@@ -0,0 +1,11 @@
+$basePackage="Microsoft.AppCenter"
+$version=(Select-String -Path windows/nuspecs/nuget/AppCenter.nuspec -Pattern "version>(.*)<").Matches.Groups[1].Value
+foreach ($packageSuffix in @("", ".Analytics", ".Crashes", ".Distribute", ".Push")) {
+  $package = "$basePackage$packageSuffix"
+  $url = "https://msmobilecenter.pkgs.visualstudio.com/_apis/packaging/feeds/$env:NUGET_FEED_ID/nuget/packages/$package/versions/$version/content"
+  $password = [System.Text.Encoding]::UTF8.GetBytes("appcenter:$env:NUGET_PASSWORD")
+  $password = [System.Convert]::ToBase64String($password)
+  $webClient = New-Object System.Net.WebClient
+  $webClient.Headers.Add("Authorization","Basic $password")
+  $webClient.DownloadFile($url, "$pwd/$package.$version.nupkg")
+}