Implement onboarding plugin for wfx

Details on the plugin:

• The plugin only interecepts the request if the client ID is set as "onboarding" and respond back without allowing the request to reach wfx. Else it lets the request pass through to wfx.
• Creates a onboarding swu file with configuration file and postinst scripts.
• Creates an DAU workflow instance (Job).
• Respond with Job ID for onboarding job to SWUpdate.

The idea is from a discussion linked here