Add short step reference for OCI image step

Problem

There is no short way of referencing an OCI image step in steps.

The only way to reference an OCI image is by using the long form:

job:
  run:
    - name: run_promoted_image
      step:
        oci:
          registry: registry.gitlab.com
          repository: my-project/my-repository/my-image
          tag: "1"
          dir: /steps/my-step
          file: step-file.yml

Proposal

A short-form should be made available for referencing OCI images. The following proposal would compile into the step reference in the description.

job:
  run:
    - name: my_step
      step: oci://registry.gitlab.com/my-project/my-repository/my-image:latest/-/steps/my-step/step-file.yml

Breaking changes

Currently, using step: gitlab.com/... would attempt to fetch a step from a Git repository. The default should be changed to load an OCI image. Git support should either be removed or a new way to load steps from Git should be used (e.g. step: git://gitlab.com/...)