Add internal events definitions validation CI job

What does this MR do and why?

Adds a new CI configuration file .gitlab/ci/events-validation.gitlab-ci.yml that wires the internal_events_validation CI component into the gitlab-org/gitlab pipeline.

How to verify

See the existing pipeline where I verified the new jobs https://gitlab.com/gitlab-org/gitlab/-/pipelines/2483936465/failures by pushing non valid event definitions.

Related

• Validator MR (unreleased): gitlab-org/analytics-section/analytics-instrumentation/event-definitions-validator!5 (merged)
• CI component: https://gitlab.com/gitlab-org/analytics-section/analytics-instrumentation/ci-components

MR acceptance checklist

This MR meets the definition of done.