Flink REST server throws exception on unknown fields in RequestBody

As FLIP-401 and FLINK-33268 mentioned, when an old version REST client receives response from a new version REST server, with strict JSON mapper, the client will throw exceptions on newly added fields, which is not convenient for situations where a centralized client deals with REST servers of different versions (e.g. k8s operator).

But this incompatibility can also happens at server side, when a new version REST client sends requests to an old version REST server with additional fields. Making server flexible with unknown fields can save clients from backward compatibility code.