Returns a single audienceList or segment entity available for the given application,
identified by its id, along with the GCS path of the CSV that represents it.

Since audienceList and segment ids come from independent sequences, the same numeric
id could exist for both entity types. entityType is required to remove that ambiguity
and restrict the lookup to the intended entity (the GET /applications/{appId}/audiences
list already returns entityType for every item, so callers always have it available).

gcsPath is only included in the response when the request is an internal request (e.g.
api-gateway internal processes / other channel services); it is omitted for any other caller.

Intended to be consumed by other channel services (email, sms, push, inapp, chat...) so
they can pull the up-to-date customer list without duplicating data.