Add lifetime back

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/event-schemas/examples/m.call.negotiate.yaml

@@ -5,6 +5,7 @@
     "version" : "1",
     "party_id": "67890",
     "call_id": "12345",
+    "lifetime": 10000,
     "description": {
       "type" : "offer",
       "sdp" : "v=0\r\no=- 6584580628695956864 2 IN IP4 127.0.0.1[...]"

data/event-schemas/schema/m.call.negotiate.yaml

@@ -59,8 +59,15 @@ properties:
         required:
         - type
         - sdp
+      lifetime:
+        type: integer
+        description: The time in milliseconds that the invite is valid for.
+          Once the invite age exceeds this value, clients should discard it.
+          They should also no longer show the call as awaiting an answer in the
+          UI.
     required:
     - description
+    - lifetime
   type:
     type: string
     enum: