Added a new listener for events related to Client

[neenapj]

No description provided.

[jamezp]

I think the ClientListener should be an interface with default no-op methods. Something like:

public interface ClientListener {
    
    default void connected(final Client client) {
         // no-op
    }
    
    default void closed(final Client client) {
         // no-op
    }

    default void connectionFailed(final Client client, final Throwable cause) {
         // no-op
    }

    default void reconnecting(final Client client, final Throwable cause) {
         // no-op
    }
}

We don't really want to use an enum because it can make it migrations more difficult. Using an interface with no-ops by default allows implementors to implement what they want and not fail if a new enum is not handled.