(2.7) Consider adding method setDefaultUseWrapper in XmlMapper

[cowtowncoder]

Currently the only way to configure default setting of "use wrapper element for Lists" is via XmlModule.
This is because actual setting needs to be propagated to AnnotationIntrospector implementation.

But since introspector may be changed after the fact (and, if we choose to, can actually make setting in introspector mutable), it may be possible to expose this via XmlMapper as well; even if not quite as efficiently. This may be more usable for users, however, especially if mapper is injected as a dependency, and thereby adding such option makes sense, it is not necessarily fully redundant.

[cowtowncoder]

Added setDefaultUseWrapper in XmlMapper for 2.7.