Replace OpenAPI RePreZen Parser

[maybeec]

The open api parser of reprezen continues to be quite instable and fragile and seems to be discontinued. As swagger-core parser is now supporting OpenAPI v3, we should move to swagger-core, which has a much more stable community.

[maybeec]

Please check migration of openapi parser to newest version of https://github.com/networknt/openapi-parser

[maybeec]

I just checked shortly and have seen some breaking test as some of the previously working openapi test files didn't work anymore. Please analyze and check whether it's some incompatibility of openapi versions or some incorrect specification of the test file.