Add support for `@JsonMerge` to allow "deep update"

[cowtowncoder]

Currently existing values of properties are not taken into consideration, with the rare case of "setterless-getters" where this is done for Collection types if no setter exists.
But it would be useful to allow this for Maps and POJOs as well.

Proposed mechanism is to allow use of:

public class POJO {
    @JsonSetter(merge=OptBoolean.TRUE)
    public List<String> names = new ArrayList<>();
    {
         names.add("first");
    }

In addition to per-property annotation, it should also be possible to specify "config overrides" using:

objectMapper.configOverrides(MyPOJO.class)
     .setter(JsonSetter.Value.from(.....));

which would specify default merge/no-merge settings for properties of specified type.
It would probably also make sense to allow setting fully global default, similar to ability to set "serializationInclusion" global defaults (with JsonInclude.Value).

[cowtowncoder]

Initial implementation now exists, supporting POJOs, Collections, Maps, reference types.