Added multi-part user data processing.
1. Added PluginSet class which loads user data plugins from plugins\windows\userdata-plugins folder
2. Added the following handlers:
    a) cloud-config handler (does nothing at this stage)
    b) part-handler (installs part handler script and uses it for each next part)
    c) head user-data (processes user-data sent via Heat)
    d) shell-script (executes .cmd, .sh, .ps1, .ps scripts. uses file extension to understand how to execute script)