[EngSys] Modernize check "Swagger Avocado"

[mikeharder]

• Migrate check from openapi-alps to GitHub Action in specs repo
  • Ideally lift-and-shift with minimal changes to start collecting data as quickly as possible
• Enable check in "Preview" mode
  • Run on all new PRs
  • May be able to retrospectively run on existing PRs
• Disable check in openapi-alps
  • No longer needed, since check is replaced by action, and check was already optional
• Monitor status of check to determine pass/fail percentage and fix common issues
• When confident, make GitHub Action required, and disable check in openapi-alps
• Migrate @azure/avocado source code to eng/tools