Document the design for the vic-machine API

This document proposes a design for a comprehensive vic-machine API,
the implementation of which will be tracked by #6116.

Subsets of this API (tracked by #5721, #6123, and eventually others)
will be implemented incrementally, and the design will be revised as
those efforts progress to reflect changes to the long-term vision.

Implementation status will be captured in GitHub, not this document.