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.
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