SubstrateClient connection event handlers

[RostislavLitovkin]

Problem: I am currently unable to inform users in my application about the connection status of the SubstrateClient.

Solution: Introduce EventHandler<T>s that are fired whenever client connects, disconnects, or is reconnecting...

These 5 event handlers would be useful: onConnect onDisconnect onReconnecting onReconnected onReconnectFailed