jobs: Add gitlab triggerable jobs configuration for swh packages

For each swh package, add a "gitlab builds" job that will be triggered when pushing commits or creating merge requests on a swh package repository hosted on a swh GitLab instance.

Those jobs are based on the use of Jenkins integration on the GitLab side and on the use of the GitLab plugin on the Jenkins side.

Depends on D8961

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Migrated from D8981 (view on Phabricator)

mentioned in merge request !145 (closed)

lgtm

one question inline.

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