async support for ClientSessionDelegate in the FFI

At the moment the FFI bindings for interacting with the keychain are both blocking (non-async) functions. Unfortunately the host language I am using this with (react-native) features an async keychain, presenting some challenges, especially with 'reloading' the keychain when dealing with OIDC locks.

I've locally made a change to use an async_trait here and it seems to be working just fine. It required some adjustment of the traits in matrix_sdk::authentication as well since async functions cannot be passed quite as easily, but its not too bad.

```
#[matrix_sdk_ffi_macros::export(callback_interface)]
#[async_trait::async_trait]
pub trait ClientSessionDelegate: Sync + Send {
async fn retrieve_session_from_keychain(&self, user_id: String)
-> Result;
async fn save_session_in_keychain(&self, session: Session);
}
```

Would be happy to package this up and merge it if there is appetite for such a change.

Start by locating the FFI ClientSessionDelegate and then read the related traits in matrix_sdk::authentication. Check how the async_trait change affects session retrieval and saving from the react-native host; done means both keychain operations can be called asynchronously through the FFI without breaking the adjusted authentication traits.

Written by the indexing model from the issue text.