Add initial Go command line interface

Small MVC to add Go command line interface:

• Adds cli/cmd/gitlab-orchestrator/main.go as the stub for the CLI
• Adds Makefile for common tasks
• Adds go.mod and go.sum to track Go modules (dependencies)
• Updates .gitignore to ignore generated binary path

Closes #277 (closed)

Testing

1. cd cli
2. Run make help to see a list of available options
3. Run make or make build to build the Go binary
4. Run ./orchestrator help to see the help output