refactor tool to be more Cloud Native

A CLI tool is useful for local development but Cloud Native scheduling tools such as Cloud Scheduler work by sending a HTTP request on a defined schedule.

Rework the tool so that the execution engine is moved to a dedicated submodule within the package and the CLI tool becomes a (light) wrapper around it. Update the Docker packaging to provide a simple web application which supports triggering the same commands as exposed by the CLI wrapper but via HTTP POST endpoints.