Expose CoalescedLocation

[jc-harrison]

Once #4584 is addressed, we'd like to be able to use FlowAPI to run a query which uses a query of this new kind as the "fallback location" in a CoalescedLocation query. This requires us to Expose CoalescedLocation and the new query class from #4584 through the API - either as two separate query schemas (with the new query kind an allowed input to 'coalesced_location') or combined into a single exposed query. Add the new query schema(s) to ReferenceLocationSchema, and add corresponding flowclient query specs.

[jc-harrison]

I suggest that for now we roll the new query from #4584 into the exposed CoalescedLocation endpoint, so the parameters would be:

• preferred_location (e.g. a MajorityLocation query)
• fallback_location (e.g. a different MajorityLocation query)
• subscriber_location_weights (e.g. a LocationVisits query)
• weight_threshold (a number)
  The query would then join fallback_location to subscriber_location_weights, subset to weight > weight_threshold, and use that as the fallback location in a CoalescedLocation query.