Auto-detect Jackson XML and JAXB in RestClient

[xehpuk]

Closes gh-34000

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[bclozel]

I don't think this is an oversight. Requesting XML using HTTP clients is a bit out of fashion so we don't intend to add this by default.

[xehpuk]

Isn't adding Jackson XML to the classpath opting-in? Not having it is already the default.

[bclozel]

Automatic classpath detection and general support are different things. You can already configure XML support in RestClient quite easily. Whether we automatically configure the converter in RestClient and the ordering matters. Community members have recently complained that this mechanism is too agressive and that libraries can sometimes be on your classpath against your will (see #32384). For more opinionated preferences, maybe we should defer to Spring Boot.