[ws client] RegisterNotification support (#303)

* Rename NotifResponse to SubscriptionResponse to make room for new impl

* Add support for on_notification Subscription<T> types

* Fix handling of NotificationHandler in manager

* cleanup

* Implement NotificationHandler to replace Subscription<T> and clean up plumbing

* More cleanup

* impl Drop for NotificationHandler

* Address pr feedback #1

* ws client register_notification pr feedback 2

* Fix doc

* fix typo

* Add tests, get NH working

* More cleanup of String/&str

* fix doc

* Drop notification handler on send_back_sink error

* ws client notification auto unsubscribe when channel full test