Provide OpenAPI specification

[rmannibucau]

OpenAPI (v3) is the de facto standard and superseeds Swagger v2 - for example, industry leaders as Microprofile, Kubernetes use OpenAPI as specification format.
Since OpenAPI is the up to date REST specification format and this project provides "Open API", it would be saner and more consistent with the current industry ecosystem to provide openapi.json instead of swagger.json files.
This issue is therefore about bulk converting all the swagger files to openapi format.