add private function to publish nuget package.

Author: Benny1007

src/PowerShellGet/private/functions/Publish-NugetPackage.ps1

@@ -0,0 +1,79 @@
+function Publish-NugetPackage {
+    [CmdletBinding()]
+    Param(
+        [Parameter(Mandatory = $true)]
+        [string]$NupkgPath,
+
+        [Parameter(Mandatory = $true)]
+        [string]$Destination,
+
+        [Parameter(Mandatory = $true)]
+        [string]$NugetApiKey,
+
+        [Parameter(ParameterSetName = "UseNuget")]
+        [string]$NugetExePath,
+
+        [Parameter(ParameterSetName = "UseDotnetCli")]
+        [switch]$UseDotnetCli
+    )
+
+    $Destination = $Destination.TrimEnd("\")
+
+    if ($PSCmdlet.ParameterSetName -eq "UseNuget") {
+        $ArgumentList = @('push')
+        $ArgumentList += "`"$NupkgPath`""
+        $ArgumentList += @('-source', "`"$Destination`"")
+        $ArgumentList += @('-apikey', "`"$NugetApiKey`"")
+        $ArgumentList += '-NonInteractive'
+
+        #use processstartinfo and process objects here as it allows stderr redirection in memory rather than file.
+        $processStartInfo = New-Object System.Diagnostics.ProcessStartInfo
+        $processStartInfo.FileName = $NugetExePath
+        $processStartInfo.RedirectStandardError = $true
+        $processStartInfo.RedirectStandardOutput = $true
+        $processStartInfo.UseShellExecute = $false
+        $processStartInfo.Arguments = $ArgumentList
+
+        $process = New-Object System.Diagnostics.Process
+        $process.StartInfo = $processStartInfo
+        $process.Start() | Out-Null
+        $process.WaitForExit()
+
+        if (-Not ($process.ExitCode -eq 0 )) {
+            $stdErr = $process.StandardError.ReadToEnd()
+            throw "nuget.exe failed to push $stdErr"
+        }
+    }
+
+    if ($PSCmdlet.ParameterSetName -eq "UseDotnetCli") {
+        #perform dotnet pack using a temporary project file.
+        $dotnetCliPath = (Get-Command -Name "dotnet").Source
+
+        $ArgumentList = @('nuget')
+        $ArgumentList += 'push'
+        $ArgumentList += "`"$NupkgPath`""
+        $ArgumentList += @('--source', "`"$Destination`"")
+        $ArgumentList += @('--api-key', "`"$NugetApiKey`"")
+
+        #use processstartinfo and process objects here as it allows stdout redirection in memory rather than file.
+        $processStartInfo = New-Object System.Diagnostics.ProcessStartInfo
+        $processStartInfo.FileName = $dotnetCliPath
+        $processStartInfo.RedirectStandardError = $true
+        $processStartInfo.RedirectStandardOutput = $true
+        $processStartInfo.UseShellExecute = $false
+        $processStartInfo.Arguments = $ArgumentList
+
+        $process = New-Object System.Diagnostics.Process
+        $process.StartInfo = $processStartInfo
+        $process.Start() | Out-Null
+        $process.WaitForExit()
+
+        if (-Not ($process.ExitCode -eq 0)) {
+            $stdOut = $process.StandardOutput.ReadToEnd()
+            throw "dotnet cli failed to nuget push $stdOut"
+        }
+    }
+
+    $stdOut = $process.StandardOutput.ReadToEnd()
+    Write-Verbose -Message $stdOut
+}