Support staged rollout of migration to Rust Crypto (#12184)

* Rust migration staged rollout

* Phased rollout unit tests

src/settings/Settings.tsx

@@ -96,6 +96,7 @@ export enum Features {
     VoiceBroadcastForceSmallChunks = "feature_voice_broadcast_force_small_chunks",
     NotificationSettings2 = "feature_notification_settings2",
     OidcNativeFlow = "feature_oidc_native_flow",
+    // If true, every new login will use the new rust crypto implementation
     RustCrypto = "feature_rust_crypto",
 }
 
@@ -503,6 +504,13 @@ export const SETTINGS: { [setting: string]: ISetting } = {
         default: false,
         controller: new RustCryptoSdkController(),
     },
+    // Must be set under `setting_defaults` in config.json.
+    // If set to 100 in conjunction with `feature_rust_crypto`, all existing users will migrate to the new crypto.
+    // Default is 0, meaning no existing users on legacy crypto will migrate.
+    "RustCrypto.staged_rollout_percent": {
+        supportedLevels: [SettingLevel.CONFIG],
+        default: 0,
+    },
     "baseFontSize": {
         displayName: _td("settings|appearance|font_size"),
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,