Make simple_request::Client generic to the executor

Part of https://github.com/serai-dex/serai/issues/682.

We don't remove the use of `tokio::sync::Mutex` now as `hyper` pulls in
`tokio::sync` anyways, so there's no point in replacing it. This doesn't yet
solve TLS for non-`tokio` `Client`s.

substrate/client/src/serai/mod.rs

@@ -1,7 +1,7 @@
 use thiserror::Error;
 
 use async_lock::RwLock;
-use simple_request::{hyper, Request, Client};
+use simple_request::{hyper, Request, TokioClient as Client};
 
 use scale::{Decode, Encode};
 use serde::{Serialize, Deserialize, de::DeserializeOwned};