Allow providing a user_data script on Windows that doesn't overwrite the default user_data script

🗣️ Foreword

Thank for taking the time to fill this feature request fully. Without it we may not be able to , and the issue may be closed without resolution.

:person_frowning: Problem Statement

When creating a Windows instance if I specify a user_data script then that script is used instead of the default that enables winrm and adds a non-administrator user specified in the username transport options to the Administrator's User Group. As a result winrm is not enabled and kitchen can't connect to the instance.

❔ Possible Solution

A custom_user_data parameter could be used that is then included in the final user_data Powershell script:

kitchen-ec2/lib/kitchen/driver/ec2.rb

Lines 709 to 710 in 0c307ab

    
                   #{base_script} 
        
                   #{custom_admin_script}

⤴️ Describe alternatives you've considered

I've considered copying the default user_data script and adding my own modifications.

➕ Additional context

Add any other context about the problem here. e.g. related issues or existing pull requests.

I've resorted to copying the default user_data script and adding my own modifications but now I'll have to keep tabs on the default user_data in case it ever changes.

Good idea for an improvement.