Introduce Franchise-level crawlers

[fcamblor]

Franchises like Devoxx/Voxxed which have a lot of yearly recurring conferences can't be autonomous when it comes to declaring their event in Voxxrin Crawlers (for each event, they need to rely on a Voxxrin Administrators to declare a new crawler entry)

Idea of this is to introduce a new family-crawlers-provider firestore collection which is declared once for all per franchise by Voxxrin Administrators.

Having an entry on this collection will allow to call a new POST /api/family-crawlers-providers/{{familyCrawlersProviderId}}/requestFamilyCrawlersRefresh endpoint (secured by a familyOrganizer-type token)
(a daily CRON might also automatically call this endpoint)

This endpoint will:

• Rely on /family-crawlers-provider/:familyCrawlersProviderId descriptor firestore entry
• This firestore document will provide an implementation type allowing to tell which family crawler provider implementation to take
• Devoxx/Voxxed implementation will look for events declared on https://www.devoxxians.com/api/public/events/future/devoxx endpoint and, for each of them, will create (if not already exists) a new /crawlers/{crawlerId}
• /crawlers/{crawlerId} entries should now have a new metadata field, allowing to pass some specific data to the crawler (typically, to avoid having to create a new JSON descriptor for every events if we consider composable descriptors Introduce Crawler descriptors composition #111)