Add namespace property for @JsonProperty (for XML module)

[cowtowncoder]

Currently "namespace" aspect for XML module is handled with separate set of XML-specific annotations (which in itself would be fine) but also with variation of AnnotationIntrospector. This leads to problems with handling, including otherwise unnecessary coupling by XML module to JAXB annotations module.

To reduce coupling a bit it would make sense to allow optionally specifying namespace for @JsonProperty annotation -- handling already allows this (at databind level), and default of "" is assumed already.
Note that @JsonRootName already allows specifying namespace this way.

So for 2.12 let's add ability to use Jackson's standard annotations in most cases.