Add an XML option to postgresql12-contrib

Avoids libxml2/libxslt for systems that don't need XML support.
Although there was substantial interest in the PR to backport to
10, I've only done pgsql12 for now, as I don't want to interfere
with the effort to make pgsql11 the default. If things work well
here, it can be backported to earlier versions as well.

PR: 239638
Approved by: maintainer timeout (2 weeks)