`zetacore` : fix events for add observer

[kingpinXD]

Message AddObserver currently emits events when a new observer is not added, this should be refactored to make sure this event is only emitted when a new observer gets added to the set

[kingpinXD]

In my opinon its just better to refactor this message and break it down into two different messages

• AddSignerNodeAccount
• AddObserverToObserver

The AddNodeAccountOnly flag is just extra logic we dont need