refactor: new ClientBuilder

[carlocorradini]

Fix #112

let (client, actor) = Client::builder().build(connection).await.unwrap();

Client::build and builder::ClientBuilder are deperecated.
I've added #[deprecated(since = "0.11.0", note = "use Client::builder() instead")] (update since if necessary)
Moreover, connection is needed only when calling subscribe or build having the advantage that the overall implementation is much simpler since there is no need for:

• connection: Box<dyn ObjectSafeConnection>

• impl IntoFuture for ClientBuilder {
      type Output = Result<(Client, ConnectionActor), Error>;
  
      type IntoFuture = future::Boxed<Self::Output>;
  
      fn into_future(self) -> Self::IntoFuture {
          Box::pin(self.build())
      }
  }

What do you think?

[carlocorradini]

Add #[allow(deprecated)] in builder.rs or?