Add a feature to support writing `xsi:nil` attribute for `null` values

[cowtowncoder]

(note: follow-up for #354

Now that Jackson processes xsi:nil on reader side, with 2.10, it would make sense to also emit those declarations in some cases. Since this is additional content (attribute) that some other systems might not recognize (or users not want), this should be added as ToXmlGenerator.Feature, defaulting to false (i.e. do not write xsi:nil).

[Migwel]

I'd like to give this one a go, @cowtowncoder . Any suggestion on how to tackle this? I see that we have a method com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator#writeNull but I'm not sure (yet) if the change should happen there or also in the xmlWriter itself

[cowtowncoder]

@Migwel I fixed one typo in description: feature to add would be in generator (ToXmlGenerator).

But yes, I think that would be the method to modify. Looking at it, I think first cases -- attribute, "unwrapped" -- probably need to be left as is (i.e. cannot make use), and it's the 3rd case that matters.
In that section, instead of writing empty element, would have to change it to write start element, write namespaced attribute (xsi:nil, with proper namespace passed, so using Stax write method that take local name, prefix and URI all), and then write end element.
Tricky part would be that of indentation, not sure how to support pretty-printing (indenting) case -- probably would (eventually?) need to change XmlPrettyPrinter to allow for this case. But maybe start with simpler part.

One possible complication is that while Stax generator will handle namespace declaration for xsi, it may get unwieldy with multiple declarations. But I guess we'll see, starting with what can be done.