Introduce Crawler descriptors composition

[fcamblor]

It would be nice if crawler descriptors might compose with other JSON descriptor files, so that common properties across yearly events and franchise events are declared only once for all

This would allow to make changes at different levels.

For instance, for Devoxx/Voxxed maintainers, we might have somehting like:

{
    "_descriptorCompounds": [
        "https://path/to/devoxx+voxxed/properties.json", // for instance, might declare `features .ratings.bingo.choices` to have an homogenous set of choice across all devoxx/voxxed events
        "https://path/to/devoxx/properties.json", // for instance, will declare `eventFamily=devoxx` and `theming` entries
    ],
    "cfpBaseUrl": "https://devoxxpl24.cfp.dev",
    // etc. every event-specific entries stay in current descriptor
}

Some problematics to take into consideration:

• order of _descriptorCompounds is important: every compounds will overwrite properties from previous compounds
• for nested object/arrays entries, we should think about how overriding works: is it a "merge" (merging array entries) or an overwrite ? answer might be different depending on the path (we might have mergeable bingo choice for instance, but consider that talkTracks, for instance, should not be composable.