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/IConfigOptions.ts

@@ -191,6 +191,8 @@ export interface IConfigOptions {
         description: string;
         show_once?: boolean;
     };
+
+    use_rust_crypto_sdk?: boolean;
 }
 
 export interface ISsoRedirectOptions {