Version 2: Bring back `SpecVersionDetector` / make `SpecificationVersionDetector` public

[sschuberth]

Looks like SpecVersionDetector, of which we're using the detect(JsonNode) function, has gone version 2 without a substitute in the public API.

However, there is a test class called SpecificationVersionDetector now. Any chance to make that public by moving it from the test to the main source set?

[justin-tay]

Duplicate of #1206