Support OpenAPI 3.0 as source data

[kazuhitoyokoi]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

I added code to support OpenAPI 3.0 as one of the source input data.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 65.161% when pulling af642a7 on kazuhitoyokoi:master into db63b60 on node-red:master.