rpc middleware: introduce separate response types

This could be a followup thing to try out but: I wonder whether it's worth now having a separate response type for each call, so that there is no enum?

eg

```rust
type CallResponse = ...
type BatchResponse = ...
type NotificationResponse = ...
```

Originally posted by @jsdw in https://github.com/paritytech/jsonrpsee/pull/1521#discussion_r2033435123_
            