WIP - xmlvalidate implementation from TODO list

Hello!

I am likely one of the few people still using XML, but I noticed XSD schema
validation is still a TODO on postgresql, which I have some personal use
cases for.

In this patch I attempt to implement XMLVALIDATE with the already in use
libxml following a version I got my hands on of the SQL/XML standard of
2016.

In short, I had to add the ACCORDING word to comply with it and completely
ignored the version that was already in the src that fetches arbitrary
schemas (it refers to validations of dtds, which is another more
troublesome TODO).

I had problems running the regression tests on my machine, so I could only
test the feature by spawning a modified instance of postgresql and issuing
queries through psql, therefore I am marking it as WIP. If anyone can
assert the tests pass, I would be glad.

Also, this is my first patch, so I might have not followed standard
practices as best as I could, so please pay particular attention to that on
review.

Cheers,
Marcos Magueta.