Make WakuRelay API closer to EventEmitter

[D4nte]

This is a feature request

Problem

The WakuRelay API implements the observer design pattern.
It is reminiscent of NodeJS's EventEmitter, which is a popular interface in the JS that also implement the observer design pattern:

WakuRelay	EventEmitter
send	emit
addObserver	on
deleteObserver	removeListener

However, WakuRelay's API is very limited in comparison to EventEmitter.

See https://nodejs.org/api/events.html for details.

When implementing a socket-like usage of js-waku for WebRTC, I have found the lack of once and removeAllListeners limiting.

Note: the API does not use the same function names because WakuRelay extends EventEmitter by transition.

Proposed Solutions

Implement the following methods on WakuRelay:

WakuRelay	EventEmitter equivalent
addOnceObserver	once
removeAllObservers	removeAllListeners

Feel free to implement all methods equivalent but having check I am uncertain there other methods worth it apart from prepend and count.

[fryorcraken]

once does not really work because one cannot trust that the next message arriving on a given content topic is valid and not spam/garbage.

The work done in waku-org/js-noise#14 enables us to already improve the API for a concrete use case.

Iceboxing for now until we have a concrete need or request of API from developers.

[weboko]

@fryorcraken I think current issue lost it's actuality to #1635 so it should be safe to close this one

[fryorcraken]

#1635 is more around wrappers.

However, happy to close this and just track work based on recent/popular developer feedback.