ci: add OpenAPI linting and validation

[uniqueg]

Add an additional GitHub actions workflow for OpenAPI linting and validation.

Fixes https://github.com/ga4gh/task-execution-schemas/issues/178

The workflow contains the following three jobs, executed in parallel:

• Action OpenAPI Lint Action, based on Redocly CLI:
  • Lints OpenAPI definition at openapi/task_execution_service.openapi.yaml
• Action Swagger Editor Validator, based on a call to Swagger Editor:
  • Validates the OpenAPI definition at openapi/task_execution_service.openapi.yaml
• Action OpenAPI Diff, based on OpenAPI Diff:
  • Only runs for pull requests!
  • Creates diff between openapi/task_execution_service.openapi.yaml definition in head and base/target branch
  • Highlights breaking, non-breaking and unclassified changes
  • Note that check will fail on changes (at least on breaking changes) and GitHub Actions currently does not support allow-failure (see issue)

[uniqueg]

@kellrott or @aniewielska: Both the lint and diff tests completed successfully (or as expected) when tested in my fork. I suspect the failing checks are due to the PR coming from a fork. Perhaps you could add the new workflow file from within this repo and see if tests pass. Alternatively, you could give me write permissions to this repo so that I can try it myself.

See relevant tests results in my fork:

• Latest push to head branch for this PR
• Test PR against branch for this PR to check behavior of diff job on breaking changes

[uniqueg]

Superseded by actions/toolkit#181