Add serialization support for `@JsonUnwrapped` with `ObjectNode`

[cowtowncoder]

Currently @JsonUnwrapped has no effect on ObjectNode (or JsonNode) valued property.
Although in general JsonNode is to be handled as-is, with minimal changes (unlike POJOs), this seems like one case where property-annotation would sometimes make sense.

Specifically, it'd be great to allow sort "any-getter" alternative, where entries of an ObjectNode were written as if they were the properties to include without wrapper. Something like, say,

public class Root {
   @JsonUnwrapped
   public ObjectNode values; // having values of: { "a" : 1, "b" : 3 }
}

would get serialized as

{"a":1,"b":3}

instead of what happens currently

{  "values":{"a":1, "b":3}}

NOTE: this issue is specifically for allowing serialization -- supporting deserialization side would have separate challenges.

[cowtowncoder]

Note: the specific immediate challenge here is that instead of there being special JsonNodeSerializer (or subtypes), JsonNode implements JsonSerializable so actual classes include serialization logic and there is just SerializableSerialiazer thin wrapping layer.

Given this, it'd probably be necessary to change things slightly to, perhaps:

1. Create specific SerializableSerializer alternative for JsonNode types, to handle JsonSerializer.unwrappingSerializer() call
2. Figure out how to dynamically deal with requests to "unwrap"

[cowtowncoder]

Quick note: alternatively we could consider allowing ObjectNode (or JsonNode) to be used with @JsonAnyGetter.
This might be a better way to go, given that #3394 (include in 2.14) allows use with @JsonAnySetter.

Actually... given there is already #3604 requesting just that, I will close this issue as "won't fix".