Add AnnotationIntrospector.XmlExtensions interface for decoupling javax dependencies

[cowtowncoder]

(note: needed by FasterXML/jackson-dataformat-xml#462)

So. Javax/Jakarta fiasco has made it untenable to have JAXB dependencies from other components, and this specifically affects Jackson XML format module. To sever this dependency we'll have to add a little bit of format-specific stuff in databind: essentially an add-on for AnnotationIntrospector that other components can implement/depend on.
Doing this is not ideal but seems like the simplest way to remove dependency between JAXB annotation handling and XML format module.