add basic xmllint ci template

It's really basic so feel free to suggest any improvement.

For now, it will run on all the xml files in the repo, but I guess we can reduce it to only the updated ones using git diff --name-only origin/$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME | grep '\.xml$' | xargs xmllint (according to https://forum.gitlab.com/t/ci-cd-pipeline-get-list-of-changed-files/26847/15) and adding a rule on changes ".xml" and "**/.xml".

I haven't tested it, it's theorical.