Resolve "Run the schema-validation gem"

In this merge request, we are adding a new job to the db-testing pipeline. The goal of this job is to run the schema-validation gem.

I have created a schedule pipeline that triggers this job every Saturday. Example: https://ops.gitlab.net/gitlab-com/database-team/gitlab-com-database-testing/-/pipelines/2191540

Note: When the database testing pipeline is trigger this job will not run.

In the next merge requests, we will add a new method to the notifier.rb to consume the schema-validation artifacts. We will also remove the schema-validation directory from the project.

Closes #125 (closed)