Add a config flag to enable the rust crypto-sdk (#9759)

This PR adds an option to `config.json` which will make the js-sdk use the rust crypto sdk, instead of the libolm implementation.

To use it, you need to add something like this to `config.json`:

```
    "features": {
        "feature_rust_crypto": true
    },
```

We don't (yet) have any way to migrate a device between implementations, so the setting that was in use when you log in is persisted to the device; it is *visible* via the labs section but cannot currently be changed.

This is part of https://github.com/vector-im/element-web/issues/21972, and enables the functionality added to the js-sdk in https://github.com/matrix-org/matrix-js-sdk/pull/2969.

src/settings/Settings.tsx

@@ -44,6 +44,7 @@ import SdkConfig from "../SdkConfig";
 import SlidingSyncController from "./controllers/SlidingSyncController";
 import ThreadBetaController from "./controllers/ThreadBetaController";
 import { FontWatcher } from "./watchers/FontWatcher";
+import RustCryptoSdkController from "./controllers/RustCryptoSdkController";
 
 // These are just a bunch of helper arrays to avoid copy/pasting a bunch of times
 const LEVELS_ROOM_SETTINGS = [
@@ -491,6 +492,17 @@ export const SETTINGS: { [setting: string]: ISetting } = {
         ),
         default: false,
     },
+    "feature_rust_crypto": {
+        // use the rust matrix-sdk-crypto-js for crypto.
+        isFeature: true,
+        labsGroup: LabGroup.Developer,
+        supportedLevels: LEVELS_DEVICE_ONLY_SETTINGS_WITH_CONFIG,
+        displayName: _td("Rust cryptography implementation"),
+        description: _td("Under active development. Can currently only be enabled via config.json"),
+        // shouldWarn: true,
+        default: false,
+        controller: new RustCryptoSdkController(),
+    },
     "baseFontSize": {
         displayName: _td("Font size"),
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,

src/settings/controllers/RustCryptoSdkController.ts

@@ -0,0 +1,25 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import SettingController from "./SettingController";
+
+export default class RustCryptoSdkController extends SettingController {
+    public get settingDisabled(): boolean {
+        // Currently this can only be changed via config.json. In future, we'll allow the user to *enable* this setting
+        // via labs, which will migrate their existing device to the rust-sdk implementation.
+        return true;
+    }
+}