Add a SharedClient and SharedServer

[tomaka]

After some discussion with @jimpo, let's make the API more convenient to use.

As a first step, let's add a SharedClient and a SharedServer. See the sketches below.
As a second step, let's rename things around: RawClient/RawServer should become ClientTransport/ServerTransport, Client/Server should become RawClient/RawServer, and SharedClient/SharedServer should become Client/Server.

Sketches for the new API layers:

impl SharedClient<...> {
    pub async fn request<Ret>(&self, method: ..., params: ...) -> Ret;
    pub async fn subscribe(&self, method: ..., params: ...) -> Subscription;
}

impl Subscription {
    pub async fn next(&mut self) -> ...;
}

impl Drop for Subscription {
    ...  // unsubscribes
}

impl SharedServer<...> {
    /// Registers a request that the server must handle. Returns an error if already registered.
    pub fn register_request<Params, Ret>(&self, method: ...) -> RequestHandler<Params, Ret>;
    pub fn register_subscription<Notif>(&self, subscribe_method: ..., unsubscribe_method: ...) -> Subscription<Notif>;
}

impl<Params, Ret> RequestHandler<Params, Ret> {
    /// User must call this repeatedly and answer the requests.
    pub async fn next(&mut self) -> ...;
}

impl<Notif> Subscription<Notif> {
    pub async fn send_all(&self, to_send: Notif) { ... }
}

[tomaka]

Since we can't implement SharedClient/SharedServer in a no_std environment, we theoretically shouldn't put them in jsonrpsee-core but in a different crate.
Note that local.rs would be concerned as well.

[tomaka]

A nice property of SharedClient/SharedServer is that they don't need any template parameter.