Offer Abstract Delegators

The **API** should offer abstract implementations for both ``JsonObject`` and ``JsonArray``. These implementations will encapsluate a ``JsonObject`` and ``JsonArray`` delegator respectively , in order to leverage polymorphism.

E.g.

```java
public abstract class AbstractJsonObject implements JsonObject {
private final JsonObject delegate;

public AbstractJsonObject (final JsonObject delegate) {
this.delegate = delegate;
}

@Override
public final JsonArray getJsonArray(final String name) {
return this.delegate.getJsonArray(name);
}
//all other implementations, all delegated
}
```

Here are some examples of usage (object orientation):

Polymorphic entity: https://github.com/amihaiemil/docker-java-api/blob/master/src/main/java/com/amihaiemil/docker/RtImage.java (extends JsonResource which is the same concept).

2 Articles:

https://www.amihaiemil.com/2017/10/16/javaee8-jsoncollectors-oop-alternative.html
https://www.amihaiemil.com/2017/06/14/non-flushable-jsonobjectbuilder.html (this is for JsonObjectBuilder)

This should be an API addition, obviously, since these classes are provider-agnostic, they are abstract and do not care about any concrete implementation.

If this is wanted, I'll make a PR myself.

Start by reviewing the JsonObject and JsonArray interfaces, then compare the linked RtImage example and its JsonResource base class for the intended delegation pattern. Done means the API offers provider-agnostic abstract delegators for both interfaces, with the interface operations delegated consistently.

Written by the indexing model from the issue text.