Added exampleSetFlag to nonValidationKeyword

[ShubhamRwt]

Issue:

When using vertx, there is a keyword exampleSetFlag present in swagger which is transitive dependency of vertx . So when we run the verticle we get a warning like this :-
<onMetaSchema:338> [oop-thread-1] Unknown keyword exampleSetFlag - you should define your own Meta Schema. If the keyword is irrelevant for validation, just use a NonValidationKeyword

PR purpose :-

It adds the variable exampleSetFlag to the nonValidationKeyword in V4 since the OpenApiv3 use the Json schema v4 --> private static final JsonMetaSchema META_SCHEMA = JsonMetaSchema.builder(JsonMetaSchema.getV4().getUri(), JsonMetaSchema.getV4()).addKeyword(new NonValidationKeyword("example")).build();

[stevehu]

This is a temp solution to compensating a bug from the swagger. And it might be removed in the future as the exampleSetFlag is not a JSON Schema keyword.

I did a search and seems this issue has been resolve by the swagger team already. swagger-api/swagger-core#3702
Also, it is only a warning message in the log, it this a big concern for your team? I am OK to accept a PR as a temporary workaround for the Swagger bug and you should change the version you are using not the v4 I mentioned in the example above.