Ship a working and useful `substreams init` command

[maoueh]

We have brewing and updated code generation tool for Substreams project. Right now there is two commands for it:

• substreams alpha init
• substreams alpha codegen

The first one initial a brand new project, the second re-generate code on substreams.yaml modification.

[jubeless]

Current attached PR: https://github.com/streamingfast/substreams/pull/171