chore: improve WakuRelay subscribe return value

[gabrielmer]

Background

Waku Relay's subscribe returns a wrapped handler which we almost always discard

nwaku/waku/waku_relay/protocol.nim

Lines 386 to 388 in 6627e34

	proc subscribe*(
	w: WakuRelay, pubsubTopic: PubsubTopic, handler: WakuRelayHandler
	): TopicHandler =

the idea of returning the wrapped handler is so we can then unsubscribe to that specific handler

nwaku/waku/waku_relay/protocol.nim

Lines 435 to 440 in 6627e34

	proc unsubscribe*(w: WakuRelay, pubsubTopic: PubsubTopic, handler: TopicHandler) =
	## Unsubscribe this handler on this pubsub topic
	debug "unsubscribe", pubsubTopic = pubsubTopic
	procCall GossipSub(w).unsubscribe(pubsubTopic, handler)

which is a feature we don't use much, but Waku Relay supports.
We have to wrap the original handler passed to subscribe so it matches the proc signature that GossipSub expects from a handler.

The flow of subscribe returning a handler which we almost always discard or having to save it to pass as a parameter to unsubscribe is a bit cumbersome.

We should review whether we need/want the feature of subscribing and/or unsubscribing specific handlers, and if so, consider a simpler way.

It could be requiring the user to pass a handler in the format that GossipSub expects and avoid wrapping it ourselves, adding a name/id to each set handler so it can be referenced by the unsubscribe procedure, or any other solution that makes us avoid all the unnecessary discards and spares the user from saving a wrapped handler just in case they want to call unsubscribe.