Support OpenAPI Specifications for Mocking

What you did:
See of opeanpi/swagger specs were supported.

What happened:
OpenAPI v2 (Swagger) and OpenAPI v3 are not supported.

Problem description:

Suggested solution:

Add support for serving openapi v2 (swagger) and openapi v3 specifications. A lot of people already use openapi v2/3 to document and provide examples for their apis. There is a lack of good mocking tools around openapi, it'd be great if mockit supported serving an openapi specification. Supporting both versions of the OpenAPI format (v2/v3) would be ideal as a large amount of tooling and specs already exist around v2 (formerly swagger).

The issue names no files or tests; start by mapping mockit’s existing endpoint-serving entry point and how it handles API definitions. Define what support for OpenAPI v2 (Swagger) and v3 means for serving specifications, then verify both formats are accepted and usable for mocking.

Written by the indexing model from the issue text.