Add CryptoApi.getActiveSessionBackupVersion() (#3555)

* stub backupmanager

* Implement `CryptoApi.getActiveSessionBackupVersion`

* Revert unnecessary change

we can do this later, once we have better test coverage

* more test coverage

---------

Co-authored-by: Richard van der Hoff <richard@matrix.org>

src/rust-crypto/rust-crypto.ts

@@ -56,6 +56,7 @@ import { RustVerificationRequest, verificationMethodIdentifierToMethod } from ".
 import { EventType } from "../@types/event";
 import { CryptoEvent } from "../crypto";
 import { TypedEventEmitter } from "../models/typed-event-emitter";
+import { RustBackupManager } from "./backup";
 
 const ALL_VERIFICATION_METHODS = ["m.sas.v1", "m.qr_code.scan.v1", "m.qr_code.show.v1", "m.reciprocate.v1"];
 
@@ -82,6 +83,8 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
     private outgoingRequestProcessor: OutgoingRequestProcessor;
     private crossSigningIdentity: CrossSigningIdentity;
 
+    public readonly backupManager: RustBackupManager;
+
     public constructor(
         /** The `OlmMachine` from the underlying rust crypto sdk. */
         private readonly olmMachine: RustSdkCryptoJs.OlmMachine,
@@ -109,6 +112,7 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
         this.outgoingRequestProcessor = new OutgoingRequestProcessor(olmMachine, http);
         this.keyClaimManager = new KeyClaimManager(olmMachine, this.outgoingRequestProcessor);
         this.eventDecryptor = new EventDecryptor(olmMachine);
+        this.backupManager = new RustBackupManager();
 
         // Fire if the cross signing keys are imported from the secret storage
         const onCrossSigningKeysImport = (): void => {
@@ -780,6 +784,15 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
         await this.olmMachine.saveBackupDecryptionKey(RustSdkCryptoJs.BackupDecryptionKey.fromBase64(base64Key), "");
     }
 
+    /**
+     * Get the current status of key backup.
+     *
+     * Implementation of {@link CryptoApi#getActiveSessionBackupVersion}.
+     */
+    public async getActiveSessionBackupVersion(): Promise<string | null> {
+        return await this.backupManager.getActiveBackupVersion();
+    }
+
     ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
     //
     // SyncCryptoCallbacks implementation