Add OCI short reference parsing

This MR replaces step git short reference parsing with OCI short reference parsing.

The image reference and step file path are separated by a space. Any image reference that name.ParseReference can parse is allowed, including ports in the registry and digest instead of a tag. If not specified, the default registry is registry.gitlab.com and the default tag is latest. Similarly, if not specified, the step file defaults to step.yml at the root of the OCI image file system.

I haven't removed git long reference parsing here. That can come in a follow up MR.

Fixes Add short step reference for OCI image step (#179)

Best reviewed commit-at-a-time.