Support delayed events (MSC4140) for call widget (#12714)

The Widget API spec for delayed events is defined by MSC4157.

Also support "parent" delayed events, which were in a previous version
of MSC4140 and may be reintroduced or be part of a new MSC later.

test/test-utils/test-utils.ts

@@ -252,6 +252,10 @@ export function createTestClient(): MatrixClient {
             });
         }),
 
+        _unstable_sendDelayedEvent: jest.fn(),
+        _unstable_sendDelayedStateEvent: jest.fn(),
+        _unstable_updateDelayedEvent: jest.fn(),
+
         searchUserDirectory: jest.fn().mockResolvedValue({ limited: false, results: [] }),
         setDeviceVerified: jest.fn(),
         joinRoom: jest.fn(),