Code & Test API to Build HolonTypeDescriptor from Builder

[evomimic]

This is a backend story that will provide an initial simple implementation of the HolonDescriptorBuilder's build API to create a new HolonDescriptor from a populated HolonDescriptorBuilder. It depends upon the HolonDescriptorBuilder populated via Issue #29. Once we add properties to the HolonDescriptorBuilder, the implementation of 'build' will get significantly more complicated. For the current story, we are just trying to get the basic end-to-end builder-to-descriptor call flow implemented.

For testing purposes, we will expose this API function as an extern function so that it is callable via the Holochain Conductor. However, we intend to wrap this function behind the Uniform API + adaptor, so it will likely never actually be directly invoked from the conductor.

The design of this function follows the Prototype design pattern, with the builder object used as the prototype. The rules for populating the HolonDescriptor object are described in the Metaspace Design Spec

Definition of Done:

• HolonDescriptorBuilder's 'build' API has been implemented
• Sweetests demonstrate the correct behavior
• a Pull Request for these changes has been approved
• the feature branch for this issue has been deleted