add OpenAPI documentation

[joostholslag]

It would be nice if the available REST api's are documented using OpenAPI v3. This would facilitate auto generation of the https://github.com/SevKohler/Eos/wiki/API-endpoints page similar to EHRBase's '/swagger-ui.html' page. And it would help other projects integrate with EOS.

[SevKohler]

Maybe @yampeku finds the time when reworking it, would be helpful.
Good suggestion