Allow to exclude null properties in responses

[andrus]

For compact output let's allow users to exclude null properties of objects in responses:

AgRuntime runtime = AgRuntime
    .builder()
    .skipNullProperties()
    .build();

When enabled this response:

{ "data" : [{"id":1, "p1":1}, {"id":1, "p1":null}] }

will become this

{ "data" : [{"id":1, "p1":1}, {"id":1}] }

More granular control can be achieved by the users on their own via Jackson annotations.