middleware: RpcServiceT make it possible to use !BoxedFuture

[niklasad1]

#1215 (comment)

After #1215 was merged a new RpcServiceT which requires the Future to be boxed and because it is used for every JSON-RPC call it can significantly increase the memory usage if many layers are used.

Let's introduce a trait that makes it possible to not box all futures:

pub trait RpcServiceT<'a> {
    /// The future response value.
    type Future<'cx>: Future<Output = MethodResponse>>
    where
        Self: 'cx,
        Request: 'cx;

    /// Process the request and return the response asynchronously.
    fn call(&self, req: Request<'a>) -> Self::Future<'_>;
}