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.
