Add support for MSC3030 /timestamp_to_event (#2072)

 - `/jumptodate` slash command is being worked on in matrix-org/matrix-react-sdk#7372
 - Jump to date headers are being worked on in matrix-org/matrix-react-sdk#7339

Related to element-hq/element-web#7677

Part of MSC3030: matrix-org/matrix-spec-proposals#3030

Experimental Synapse implementation added in matrix-org/synapse#9445

src/client.ts

@@ -728,6 +728,11 @@ interface IRoomHierarchy {
     rooms: IHierarchyRoom[];
     next_batch?: string;
 }
+
+interface ITimestampToEventResponse {
+    event_id: string;
+    origin_server_ts: string;
+}
 /* eslint-enable camelcase */
 
 // We're using this constant for methods overloading and inspect whether a variable
@@ -8937,6 +8942,36 @@ export class MatrixClient extends EventEmitter {
     public async whoami(): Promise<{ user_id: string }> { // eslint-disable-line camelcase
         return this.http.authedRequest(undefined, Method.Get, "/account/whoami");
     }
+
+    /**
+     * Find the event_id closest to the given timestamp in the given direction.
+     * @return {Promise} A promise of an object containing the event_id and
+     *    origin_server_ts of the closest event to the timestamp in the given
+     *    direction
+     */
+    public async timestampToEvent(
+        roomId: string,
+        timestamp: number,
+        dir: Direction,
+    ): Promise<ITimestampToEventResponse> {
+        const path = utils.encodeUri("/rooms/$roomId/timestamp_to_event", {
+            $roomId: roomId,
+        });
+
+        return await this.http.authedRequest(
+            undefined,
+            Method.Get,
+            path,
+            {
+                ts: timestamp.toString(),
+                dir: dir,
+            },
+            undefined,
+            {
+                prefix: "/_matrix/client/unstable/org.matrix.msc3030",
+            },
+        );
+    }
 }
 
 /**