CUE SDK: add support for Dagger Engine 0.3

[sipsma]

We need to provide a way to run Dagger 0.2 plans, written in CUE, on cloak. Let's call this compatibility layer "Dagger Classic".

Goals:

• A way to run existing CUE plans without modifications

Non-goals:

• A way to write Cloak extensions in CUE
• Porting individual CUE packages to Cloak extensions
• Bindings to call any Cloak extension from CUE

[jlongtine]

I'm looking into this!