[Experimental] Added script capability to Optic

[acunniffe]

@LouManglass -- as discussed, in this PR I've implemented the scripts command to back up the demand-gen project.

Scripts are registered similarly to how you setup a task.

The most basic script looks like this:

scripts:
  do-thing: rdme push $OPENAPI_YAML --id xyz

^ But a naked command like this is hard to share with your team. Unlike an npm script there's no affordances in bash to require certain dependencies to be on your /bin. This means you also need to tell your teammates or CI pipeline to install these manually.

To address this, you can also use the expanded syntax (this is what I imagine we'd share in our blog posts to make things work automagically).

scripts:
  publish-redoc:
    command: redoc-cli bundle $OPENAPI_YAML
    dependsOn: redoc-cli
    install: npm install redoc-cli -g

Add this to your optic.yml and run apidev scripts publish-redoc

The check for redoc-cli fails and asks you to run the command again with the --install flag. This won't catch everything (if npm isn't installed you're still screwed), but it should help most teams get aligned. You can always see the failures and fallback to manual dependency resolution.

Presto! Script executed -- static docs built

Note
All scripts get OPENAPI_JSON and OPENAPI_YAML. These are paths to their respective files and are injected as env variables.

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