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

(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`).
