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>

spec/unit/rust-crypto/rust-crypto.spec.ts

@@ -569,6 +569,13 @@ describe("RustCrypto", () => {
             expect(new TextDecoder().decode(fetched!)).toEqual(key);
         });
     });
+
+    describe("getActiveSessionBackupVersion", () => {
+        it("returns null", async () => {
+            const rustCrypto = await makeTestRustCrypto();
+            expect(await rustCrypto.getActiveSessionBackupVersion()).toBeNull();
+        });
+    });
 });
 
 /** build a basic RustCrypto instance for testing