The previous approach of splitting Thor commands into multiple files caused problems when the same method name was used in multiple commands.
This also loads the Rails environment only for commands that need it. That makes the CLI boot faster for most commands or when the help should be shown. That's also why we can't use `Rails.root` in the CLI.
* Updates GitHub Actions
* Switches from `bundler/inline` to an optional group in the `Gemfile` because the previous solution didn't work well with rspec
* Adds the converter framework and tests
* Allows loading private converters (see README)
* Switches from multiple CLI tools to a single CLI
* Makes DB connections reusable and adds a new abstraction for the `IntermediateDB`
* `IntermediateDB` acts as an interface for IPC calls when a converter steps runs in parallel (forks). Only the main process writes to the DB.
* Includes a simple example implementation of a converter for now.
