add type map http::Extensions to Request and MethodResponse types

[niklasad1]

After trying to "share state" between different layers of the middleware in jsonrpsee, which is hard to
do properly.

One possibility is to inject a shared state such as Arc<Mutex<HashSet<(RequestId, MethodName, Params)>>> but it's not a good solution because the RequestId isn't necessarily unique, and it's possible that an identical RPC call may overwrite a pending call or something similar.

For instance, my own use case is that I would want to know whether a call was rate-limited or not, which was controlled by another middleware layer.

A good solution is to copy what hyper/http has for the Request/Response in the http::Extensions API so that one could just inject custom data in there, as in the following example:

impl<'a, S> RpcServiceT<'a> for Metrics<S>
where
    S: Send + RpcServiceT<'a>,
{
    type Future = BoxFuture<'a, MethodResponse>;

    fn call(&self, req: Request<'a>) -> Self::Future {
        if self.is_rate_limited() {
            req.extensions_mut().insert("rate_limited", true);
        }

        let rp = self.service.call(req);

        if rp.extensions.get("rate_limited").is_some() {
            update_metrics(true);
        } else {
            update_metrics(false);
        }
    }
}

impl<'a, S> RpcServiceT<'a> for RateLimit<S>
where
    S: Send + RpcServiceT<'a>,
{
    type Future = BoxFuture<'a, MethodResponse>;

    fn call(&self, req: Request<'a>) -> Self::Future {
        if self.guard.check() {
            req.extensions_mut().insert("rate_limited", true);
            wait_until_ready();
        }

        self.service.call(req)
    }
}