Support Jackson based XML serialization/deserialization

This commit adds support for XML serialization/deserialization based on
the jackson-dataformat-xml extension. When using @EnableWebMvc or
<mvc:annotation-driven/>, Jackson will be used  by default instead of JAXB2
if jackson-dataformat-xml classes are found in the classpath.

This commit introduces MappingJackson2XmlHttpMessageConverter and
MappingJackson2XmlView classes, and common functionalities between JSON
and XML processing have been moved to AbstractJackson2HttpMessageConverter
and AbstractJackson2View classes.

MappingJackson2XmlView supports serialization of a single object. If the model
contains multiple entries, MappingJackson2XmlView.setModelKey() should be
used to specify the entry to serialize.

Pretty print works in XML, but tests are not included since a Woodstox dependency
is needed, and it is better to continue testing spring-web and spring-webmvc
against JAXB2.

Issue: SPR-11785