powershell set default proxy for `Invoke-WebRequest`

set_proxy.ps1

# set to system default proxy
[System.Net.WebRequest]::DefaultWebProxy = [System.Net.WebRequest]::GetSystemWebProxy()
[System.Net.WebRequest]::DefaultWebProxy.Credentials = [System.Net.CredentialCache]::DefaultNetworkCredentials

# or a custom one
[System.Net.WebRequest]::DefaultWebProxy = New-Object System.Net.WebProxy("http://localhost:8080")

# or use username and password auth
[System.Net.WebRequest]::DefaultWebProxy = New-Object System.Net.WebProxy("http://localhost:8080",$true)
[System.Net.WebRequest]::DefaultWebProxy.Credentials = New-Object System.Net.NetworkCredential($user, $passwd)