Create new exec provider called pwsh in the PowerShell module

In order to support PowerShell core we need to add a new provider to the Puppet PowerShell module. This provider will be called pwsh, for example;

exec { 'do something':

   command  => 'Write-Host $PSVersionTable',

   provider => pwsh,

   require  => File['testfile'],

}

Initially;

This provider will only execute the pwsh.exe command on Windows. Later work will add cross platform, searching for the binary and PowerShell manager support.

This issue will also add some acceptance tests to test basic functionality. Later work will copy the entire existing acceptance tests to the pwsh provider.