CLI - `manifest` command

[pileks]

Add a polywrap manifest command/set of commands within the CLI.

polywrap manifest schema

• should print out the schema for the polywrap manifest file
  • should print out the schema for the current version if a manifest file already exists in the working directory?
  • should print out latest schema if no manifest file exists?

polywrap manifest migrate

• should migrate the current manifest file to the latest version

Partly related to #202