Work around Jackson XML multiple lists limitation

[RikkiGibson]

There are unreasonable limitations to Jackon's XML serialization. To use Jackson, we'd have to wrap lots and lots of list properties in a class containing a single property. I've played a bit with JAXB, the built in XML serialization, and I think we're going to achieve correct behavior with a more usable object model that way.

It'll be necessary to update JacksonAdapter to use JAXB for XML and update the generator to use JAXB annotations.

FasterXML/jackson-dataformat-xml#192

[RikkiGibson]

Setting up JAXB to replicate the necessary parts of initializeObjectMapper seems like it could be pretty challenging. Another option I've been playing with is to generate wrapper classes with the right metadata and to use the getters and setters to hide what's really going on with them.

We already generate XML list wrapper classes, so they could simply be referenced for each list property, but the getter and setter implementations would change to automatically wrap/unwrap them and the APIs would remain fairly ergonomic. It's disappointing to have to do so much trickery to get fairly simple things to work correctly with Jackson XML, but this might get us to where we need to be.

class MyItem {
    @JsonProperty("MyField")
    int myField;
}

class MyItemsWrapper {
    final List<MyItem> list;

    @JsonCreator
    MyItemsWrapper(@JsonProperty("MyItem") List<MyItem> list) {
        this.list = list;
    }
}

@JacksonXmlRootElement(localName = "MyData")
class MyData {
    @JsonProperty("SomeData")
    MyItemsWrapper someData;

    List<MyItem> someData() {
        return someData.list;
    }

    MyData withSomeData(List<MyItem> list) {
        this.someData = new MyItemsWrapper(list);
        return this;
    }

    @JsonProperty("OtherData")
    MyItemsWrapper otherData;

    List<MyItem> otherData() {
        return otherData.list;
    }

    MyData withOtherData(List<MyItem> list) {
        this.otherData = new MyItemsWrapper(list);
        return this;
    }
}

[RikkiGibson]

Solved by Azure/autorest.java#119