Mark the appservice ping response `duration_ms` field as required (#1541)

* Mark the appservice ping response duration_ms field as required

As intended in MSC2659.

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

* Add changelog

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

---------

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

changelogs/application_service/newsfragments/1541.feature

@@ -0,0 +1 @@
+Add homeserver->appservice ping mechanism, as per [MSC2659](https://github.com/matrix-org/matrix-spec-proposals/pull/2659). Contributed by @tulir at @beeper.

data/api/client-server/appservice_ping.yaml

@@ -82,6 +82,8 @@ paths:
                     The duration in milliseconds that the
                     [`/_matrix/app/v1/ping`](#post_matrixappv1ping)
                     request took from the homeserver's point of view.
+            required:
+              - duration_ms
           examples:
             application/json: {"duration_ms": 123}
         400: