Add ruby-pwsh gem
This patch adds the ruby-pwsh gem:
https://github.com/puppetlabs/ruby-pwsh
This gem is used by the puppetlabs/powershell Puppet module, so we need to install it in order to test code using that module. Without it, the tests will fail with an error:
Could not autoload puppet/provider/exec/powershell: cannot load such file -- ruby-pwsh