Add `DefaultXmlPrettyPrinter.withCustomNewLine()` to configure linefeed for XML pretty-printing

[koalalam]

Jackson version: 2.14.1

In ObjectMapper case, we can do the following:

objectMapper.setDefaultPrettyPrinter(new DefaultPrettyPrinter().withObjectIndenter(new DefaultIndenter().withLinefeed("\n")));

However in XmlMapper. the default implementation Lf2SpacesIndenter

1. Is protected
2. Doesn't seem to support a custom line separator

Does it mean we need to provide our own implementation of Indenter just to have a different line separator?

[cowtowncoder]

Seems like an oversight, thank you for reporting it.

If you have time and interest, a PR would make sense, if you end up trying writing a custom implementation anyway.