By default Jersey configures Jackson to use both Jackson annotations
and JAXB annotations when introspective types for (de)serialization.
However, the changes made in 5776d6a8 mean that Jersey no longer uses
its default ObjectMapper configuration and uses the auto-configured
ObjectMapper instead. This had the unwanted side-effect of leaving
Jersey with an ObjectMapper that only uses Jackson annotations and
ignores JAXB annotations.

This commit updates JerseyAutoConfiguration so that it will add the
JaxbAnnotationIntrospector to the auto-configured ObjectMapper for
both serialization and deserialization. It uses
AnnotationIntrospectorPair to ensure retain any existing
introspectors.

Closes gh-6310