Policy subcommand

[puerco]

This adds the initial policy subcommand. For now it supports two operations: view and create. This command will be able to do more with policies such as updating and evaluating.

Note: This needs #233 and a rebase

> sourcetool policy --help

sourcetool policy: configure SLSA source policies for a repo

The policy subcommands can be used to view, create and update the source
policy for a repository. The policy family has two subcommands:

sourcetool policy view
Shows current repository policy for a repository checjed into the SLSA community
policy repo.

sourcetool policy create
Creates a new policy for a repository and, optionally, check it into the
SLSA community repository.

Usage:
  sourcetool policy [command]

Available Commands:
  create      creates a source policy for a repository
  view        view the policy of a repository

Signed-off-by: Adolfo García Veytia (Puerco) puerco@carabiner.dev