Fixed JSON serialization to honor Jackson annotations

[nicusX]

Purpose of the change

Fix FlinkDataGenerator example: it was ignoring Jackson annotations and emitting the wrong JSON schema

Verifying this change

Run locally and on MSF

Significant changes

(Please check any boxes [x] if the answer is "yes". You can first publish the PR and check them afterward, for convenience.)

• Completely new example
• Updated an existing example to newer Flink version or dependencies versions
• Improved an existing example
• Modified the runtime configuration of an existing example (i.e. added/removed/modified any runtime properties)
• Modified the expected input or output of an existing example (e.g. modified the source or sink, modified the record schema)