Introduce the Extensibility Framework

[pvillads]

Describe the bug

The test engine must be split up such that it allows add-ins to run on top of the basic functionality supported by Power-fx. The extensibility feature must be provided by the Microsoft Extensibility Framework and used in much the same way as it is currently used in the pac cli tool. This will make it easier to negotiate an internal security review.

The interface that an extension needs to implement must be clearly identified, and well documented.

For now, only allow loading of assemblies that as signed by a microsoft authority. Later, when we have acceptance from the security experts, we will allow loading any signed assembly.