public ConfigPayloadBuilder append() {
            setAppend();
            ConfigPayloadBuilder p = new ConfigPayloadBuilder(getInnerArrayDef(name));
            elements.put(elements.size(), p);
            return p;
        }
Create a new slime object and returns its payload builder. Append the element after all other elements
in the array.

@return a payload builder for the new slime object.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java