feat: add support for a template registry

[beeman]

This PR changes the way how templates are handled by this CLI.

Currently, we have 6 hard-coded templates in this CLI. If we want to add a new template, we need to:

• we create a new template in its own repo
• ship an update to the CLI that includes that template
• manually ensure the template is kept in sync, is tested, etc.

By moving the CLI to work with a template registry, we get a few benefits:

• we create a new template in the registry
• we don't have to update the CLI but instead it will fetch the registry and list what's there
• we can have scripts that ensure templates are kept in sync, we can do tests for all templates at once.