swh.lister.gitlab: Make rate limit check optional

Samples of gitlab instance not implementing it (yet?):

• https://0xacab.org/api/v4/projects/
• https://framagit.org/api/v4/projects/
• https://salsa.debian.org/api/v4/projects/
• https://gitlab.freedesktop.org/api/v4/projects/
• https://gitlab.gnome.org/api/v4/projects/
• https://gitlab.inria.fr/api/v4/projects/

Related T989

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Migrated from D371 (view on Phabricator)