Initial implementation

The initial implementation of Step Runner.

Blueprint: https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/implementation.html

Fixes:

• Introducing CI Step Syntax (gitlab#425879 - closed) • Joe Burnett
• Execute CI step locally (gitlab#425900 - closed) • Joe Burnett
• Support CI steps outputs (gitlab#425902 - closed) • Joe Burnett • Backlog