Add unified OpenAPI spec sync workflow

[zacchua-stripe]

Why?

Unified public-preview and GA OpenAPI artifacts are now being published to the CDN for release. This PR updates creates a github action to download the unified specs, v1 spec and fixtures.

What?

• Added sync-openapi-specs.yml workflow supporting:
  • Private preview, public preview, GA, and combined GA/public-preview syncing.
  • Optional unified-spec syncing.
  • Dry-run, test-branch, and release modes.
• Added scripts to:
  • Download split specs and (optional) fixtures from the CDN.
  • Download unified specs when unified=true.
  • Create and push OpenAPI release tags.
    • GH actions triggered by unified spec workflows will have [unified] in the tag message. publish.yml will read the tag message and trigger sdk-codegen in unified mode.
• Converted publish-v1-specs.yaml into a wrapper so existing private-preview publishing workflows continue to work.
  • Will need to migrate these to sync-openapi-specs.yml then we can delete publish-v1-specs.yml
• Download split specs and fixtures under openapi/, while unified specs are downloaded under latest/ and preview/.

See Also

http://go/j/DEVSDK-3333