Create steps schema

GitLab Steps defines a minimal, core syntax on which Step Runner operates. But it also supports an extended syntax with additional "sugar" to make usage easier and more descriptive. See https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/steps-syntactic-sugar.html.

The "sugared" model is the one which will be shared between Rails / Runner / Step Runner. And the one which is used in composite step definitions. To support multiple systems, we will produce a JSON schema which defined the structure of the sugared model. And Step Runner will have a compilation process for turning the sugared model into the core proto for execution.