http client: add default backend type

[niklasad1]

After we added tower middleware support for the HttpClient one has to do:

type HttpClient = jsonrpsee::http_client::HttpClient<jsonrpsee::http_client::transport::HttpBackend>;

struct MyWrapper(HttpClient);

It would be good if folks doesn't have to do that when the default backend is used i.e, not tower middleware enabled on the client.

Such as:

/// JSON-RPC HTTP Client that provides functionality to perform method calls and notifications.
#[derive(Debug, Clone)]
pub struct HttpClient<S = HttpBackend> {
	/// HTTP transport client.
	transport: HttpTransportClient<S>,
	/// Request timeout. Defaults to 60sec.
	request_timeout: Duration,
	/// Request ID manager.
	id_manager: Arc<RequestIdManager>,
}

Code to change/fix https://github.com/paritytech/jsonrpsee/blob/master/client/http-client/src/client.rs#L236-#L245