Investigate If JsonFlatten Should Deeply Flatten

[alzimmermsft]

Investigate if JsonFlatten serialization should deeply flatten the object and its properties. The following is a comment on its handling.

// Why does this deeply flatten all sub-classes, collection, and map values?
// Should JsonFlatten simply flatten the top level property, ex
// @JsonProperty("my.flattened.value")
// private Map<String, String> value;
//
// Turns into
// {
//   "my":
//   {
//     "flattened":
//     {
//        "value": <serialized map here>
//     }
//   }
// }

[alzimmermsft]

This has been resolved by changes to support annotating fields with JsonFlatten and updates to how class level JsonFlatten is applied.

[samvaity]

According to the work/investigation on task #20150, we should aggressively/deeply flatten when@JsonFlatten is provided irrespective of the type ref parameter. Since it allows the users to specify a property hierarchy more conveniently.