Pass CryptoBackend into SyncApi (#3010)

I need to start calling back into the new rust crypto implementation from the /sync loops, so I need to pass it into SyncApi. To reduce the coupling, I've defined a new interface specifying the methods which exist for that purpose. Currently it's only onSyncCompleted.

src/client.ts

@@ -1453,6 +1453,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
     protected buildSyncApiOptions(): SyncApiOptions {
         return {
             crypto: this.crypto,
+            cryptoCallbacks: this.cryptoBackend,
             canResetEntireTimeline: (roomId: string): boolean => {
                 if (!this.canResetTimelineCallback) {
                     return false;