jobs: Add GitLab support for maven-index-exporter
Add a job that will be executed when pushing tags to the maven-indexer-exporter repository hosted on SWH GitLab.
Also rename configuration file to something more meaningful and update pipeline script to match the style of the other ones in that repository.
See https://gitlab-staging.swh.network/swh/devel/fixtures/maven-index-exporter/-/tags
Migrated from D9000 (view on Phabricator)