Better define discovery_reference fields

[jeremie-leproust]

utm-apis/utm-domains/utm-domain-commons.yaml

Lines 372 to 385 in d2e432b

	discovery_reference:
	description: >-
	An identifier that supplies information about the USS from the
	discovery perspecitve. This field is currently vague due to research
	on the appropriate discovery approach. It will be tightened when
	an operational system decides on the approach to discovery.
	For Gridded USS Discovery, this should be an x,y,z string where
	x and y are the coords of the grid and z is the zoom level. For
	example: "110,117,10"
	For FIMS Discovery, this should be the uss_instance_id which is a
	UUID.

Most common models include a discovery_reference field. In the last weeks, in the context of UTM exchanges for UPP in the Nevada test site, we faced multiple issues based on this field. Here is a short summary:

1. The field is optional, which is problematic while using the standard discovery (as opposed to the grid discovery). Based on discovery_reference optional #132, my understanding is that this field has been left optional for now. If possible, I would recommend to make it required if it also makes sense for the grid discovery
2. The documentation of the field only gives guidance for the grid discovery, but not for the standard discovery. I would recommend to make it clear that for a standard discovery, this field should be a UUID that contains the USS instance identifier.
3. Some common models don't include this field like the UASVolumeReservation, which doesn't allow the USS to know which USS instance emitted a UVR. I would recommend to include this field in this model as well.