Make `SourceHttpMessageConverter` optional

[sdeleuze]

As a follow-up to #29277, and since the inclusion of JAXB support is triggered by the classpath presence of a JAXB implementation, it makes sense to make SourceHttpMessageConverter, currently configured unconditionally, optional. That makes a big difference on native (1M of RSS reduction with current typical Spring Boot 3 arrangement, 3.4M when other usages of XML will be not be reachable by default anymore like in Tomcat or Logback) and will avoid to configure XML support by default on JVM as well without an explicit signal.

As a consequence, Spring applications using DOMSource, SAXSource, StAXSource, StreamSource or Source will need to configure SourceHttpMessageConverter explicitly in RestTemplate or Spring MVC. This will be properly documented on upgrade notes.