Add support for unwrapping a XMLElement into other XMLElement

Looking for a solution to unwrap an XMLElement into other XML Element, similar to `@JsonUnwrapped`

Example :

**Car** class
```
public class Car {
private String carModel;
private Engine engine;
}
```

**Engine** class
```
public class Engine {
private String capacity;
private String power;
}
```

Expected XML for some scenarios should be like below.
```

Audi
500L
200

```

Start with the Car and Engine examples in the issue and review the existing JAXB XML binding behavior and annotation support. Define what unwrapped output should mean for these classes and verify that the requested scenarios produce the expected XML.

Written by the indexing model from the issue text.