Support OCI short references

Changes

Support referencing a function from a step using a short reference func: [registry/]repository[:tag] [path/to/file.yml].

Example

run:
  - name: echo
    func: registry.gitlab.com/functions/echo:1.0.1
    inputs:
      message: 'Hello World'`

Reference

Add short step reference for OCI image step (#179 - closed)

The following commits have come from (or are heavily inspired by) Add OCI short reference parsing (!326 - closed):

• [included] Non functional refactoring
• [included] Method to parse short OCI image references
• [included] Also parse OCI short reference
• Refactor schema.Reference.compileOCI so it can be used in other packages
• Decorate OCI steps with the oci fetch command
• [included] Add/update tests for oci short references
• [included] Update CHANGELOG
• Pass stepName, input and env into shortRef.compileOCI
• shortReference.compileOCI returns decorated step reference
• Handle string in valueCompiler.compile
• [included] Another OCI integration tests with inputs