Add feature to include standalone='yes' in xml declaration

[duoduobingbing]

By default there is no easy way with Jackson XML to include standalone='yes' inside the XML declaration. It requires swapping out the xmlWriter which is really cumbersome because the implementations of BaseStreamWriter from Woodstox are final like RepairingNsStreamWriter. Hence to change this the entirety of e.g. RepairingNsStreamWriter needs to be copied to the project only to override BaseStreamWriter#writeStartDocument to call doWriteStartDocument(version, encoding, "yes"); instead of the default doWriteStartDocument(version, encoding, null);.

This PR adds ToXmlGenerator.Feature.WRITE_STANDALONE_YES_TO_XML_DECLARATION which is false by default.

When enabled standalone='yes' will be added to the XML declaration if including it is enabled as well:

XmlMapper mapper = new XmlMapper();
mapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);
mapper.configure(ToXmlGenerator.Feature.WRITE_STANDALONE_YES_TO_XML_DECLARATION, true);
String xml = mapper.writeValueAsString(Map.of("abc","def"));

will cause the first line to have

<?xml version='1.0' encoding='UTF-8' standalone='yes'?>

There is definetely a need for this as demonstrated by these posts:

• https://stackoverflow.com/questions/74195415/springboot-jackson-2-13-x-outputting-standalone-yes-in-xml-header
• https://stackoverflow.com/questions/52999562/add-standalone-directive-to-jackson-xml-serialization
• https://www.mail-archive.com/jackson-dev@googlegroups.com/msg00345.html

[pjfanning]

Please describe why you even need standalone='yes'. That flag is only supposed to used with DTDs - and hardly anyone uses DTDs any more.

[duoduobingbing]

Please describe why you even need standalone='yes'. That flag is only supposed to used with DTDs - and hardly anyone uses DTDs any more.

First of all thank you so much for reviewing this so quickly.

We work with a very nasty consumer on the other end which refuses to read the XML if standalone=yes is missing.