Update packages, fix example and add self test action (CI)

[okmiim]

Changes:

• @actions/http-client to 1.0.8,
  • updates only node_modules folder
  • the bot only updated packages.json file
• @APIDevTools/swagger-cli from 3.0.1 to 4.0.2
  • support for OpenApi 3.0.3
  • uses new npm scope APIDevTools (see Changelog)
• @actions/core to 1.2.4
• @actions/github to 4.0.2
• Fix example in README
  • used fixed version (... @ v1 to ...@ master)
  • rename (spelling mistake) exmaple.yaml → example.openapi.yaml
• Create workflow to run this action with example.openapi.yaml
  • tests every new push (Continuous Integration)

Note: there are many changed files due to updates inside node_modules (normally ignored in .gitignore)