hotspots: Add 'type' field to objects in 'hotspots' array.

This commit adds a 'type' field to the objects
in 'hotspots' array sent in 'hotspots' events.

We have explicitly added this field as we eventually
plan to have two type of onboarding steps, 'hotspots'
and 'one_time_notice'.

This will help clients to easily identify them.

zerver/lib/event_schema.py

@@ -334,6 +334,7 @@ def check_heartbeat(
 
 _hotspot = DictType(
     required_keys=[
+        ("type", str),
         ("name", str),
         ("title", str),
         ("description", str),