Allow adding `<?xml-stylesheet ...>` declarations (Processing Instructions)

[OndraZizka]

Describe the solution you'd like
Allow adding to the resulting XML.

Usage example
I want to serialize Java POJOs to XML in Spring REST endpoint, but include XSLT reference so that the browsers render it as HTML.

Additional context
The only similar feature I found is a flag WRITE_XML_DECLARATION.

ObjectMapper mapper = builder.createXmlMapper(true).build();
    ((XmlMapper) mapper).enable(ToXmlGenerator.Feature.WRITE_XML_DECLARATION);
    return new MappingJackson2XmlHttpMessageConverter(mapper);
}

[cowtowncoder]

(will move to correct repo)

[cowtowncoder]

Right now the only way would be to explicitly contruct either underlying XMLStreamWriter, and write both xml declaration and style sheet PI (assuming it comes before root element); or, to construct ToXmlGenerator similarly, use writeRaw() to force output of declaration.

It would be good to add new mechanism(s), I think #150 is related to this. Not sure of the best way to model this; I don't think a feature flag will work well enough, but while configuring XmlMappers is possible via new Builder API, this may be something where per-write option would be preferable.

I hope to tackle something in this space for 2.13, tagging.

[cowtowncoder]

Can now be implemented, using FasterXML/jackson-databind#5860 -- need builder-buildable XML-specific GeneratorInitializer and that can add desired output.

[cowtowncoder]

Will implement more general "allow writing XML Processing Instructions" feature which will then cover this too.

[cowtowncoder]

Intended usage:

ObjectWriter w = XmlMapper.builder().writer()
    .with(new XmlGeneratorInitializer().addPI("style-sheet", "type='text/css'")
        // possible other directives or XML Decl customization
     );
String xml = w.writeValueAsString(new MyValue);
// we get:
//
// <?style-sheet type='text/css'?>
// <value>...</value>

[cowtowncoder]

Implemented in 3.x for 3.2.0.