The runner can reference a step as a "dist" type that can be loaded by the runner helper

Problem

The runner helper will become a step-runner so that it can execute runner build-stage steps. These build-stage steps should live in the Runner repository.

When the Runner requests the runner-helper to execute a build-stage step, it should do so using the steps "dist" feature. For example:

spec:
---
run:
  - name: user_script
    step: dist://runner/build-stages/concrete
    inputs:
      name: user_script
      script: "..."

As it stands, dist steps must reside in the step-runner repository. This issue should create a mechanism for loading dist steps from the Runner repository.

Proposal

There has been a suggestion that a map of string -> steps could be used to replace the embedded fs.FS dist implementation used in step-runner.