jobs/tools: Add setup-gitlab-webhooks job

This job is responsible to setup the webhooks on swh GitLab repositories
enabling to trigger Jenkins builds when pushing changes or creating merge
requests.

It parses Jenkins jobs builder YAML configuration to get the name of the
repos with CI setup and set webhook URLs in GitLab projects through HTTP
requests to the GitLab REST API.

It is triggered at the end of the swh-jenkins-jobs-builder job after jobs
have been reconfigured.