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.

processor/monero/src/scheduler.rs

@@ -94,7 +94,7 @@ async fn signable_transaction(
           RctType::ClsagBulletproofPlus => 16,
           _ => panic!("selecting decoys for an unsupported RctType"),
         },
-        reference_block.0.block.number().unwrap() + 1,
+        reference_block.0.block.number() + 1,
         input.0.clone(),
       )
       .await?,