Correctly nest the capabilities response object

Everything is contained in a "capabilities" property, which is not represented by the schema. The example was correct.

api/client-server/capabilities.yaml

@@ -62,11 +62,15 @@ paths:
           schema:
             type: object
             required: ["capabilities"]
-            additionalProperties:
+            properties:
+              capabilities:
                 type: object
+                title: Capabilities
                 description: |-
                     The custom capabilities the server supports, using the
                     Java package naming convention.
+                additionalProperties:
+                  type: object
                 properties:
                   "m.change_password":
                     type: object

changelogs/client_server/newsfragments/1879.feature

@@ -0,0 +1 @@
+Support optional features by having clients query for capabilities.