Wrap existing CLI

[bgins]

Summary

Problem

This project will eventually rewrite the existing Fission CLI, but it starts with nothing.

Impact

We won't have a working version outside of a couple of commands for a while.

Solution

Wrap the existing CLI and shell out to it from the new Rust CLI. Gradually replace wrapped commands with handlers written in Rust. We should be able to write the interface against the wrapped CLI.

This approach will require us to download the existing binary per platform until we have completed the rewrite.