Plugin system

[chromakode]

Initial versions of sig are intentionally minimal. There's a lot of functionality I would like to add, but adding complexity to the core is undesirable. A simpler core is easier to adopt, easier to review, and easier to maintain.

By making sig extensible, we can have the best of both worlds. Here's a sketch of what it might look like:

• Discover plugins via entry points
• Configure enabled plugins with sig plugin add / sig plugin remove / sig plugin list
• Plugins can add subcommands and command line arguments
• Plugins can extend sig attest / sig verify w/ additional identity namespaces, such as "git:" (needs some design thought)