Node API: Implement publish (#91)

* Fix init signature according to Node API

See https://github.com/status-im/nim-waku/blob/master/docs/api/v2/node.md

* Update docs and example

* Node API: Basic subscribe

* Modify example with subscribe handler

* Waku Node: Implement basic publish topic

examples/v2/basic2.nim

@@ -23,10 +23,15 @@ proc runBackground(conf: WakuNodeConf) {.async.} =
   # Subscribe to a topic
   let topic = "foobar"
   proc handler(topic: string, data: seq[byte]) {.async, gcsafe.} =
-    info "Hit subscribe handler", topic=topic, data=data
+    info "Hit subscribe handler", topic=topic, data=data, decoded=cast[string](data)
   node.subscribe(topic, handler)
 
+  # Publish to a topic
+  let message = cast[seq[byte]]("hello world")
+  node.publish(topic, message)
+
 discard runBackground(conf)
+
 runForever()
 
 # TODO Lets start with Nim Node API first

waku/node/v2/wakunode2.nim

@@ -286,12 +286,13 @@ method unsubscribe*(w: WakuNode, contentFilter: ContentFilter) =
   ## TODO Implement.
 
 method publish*(w: WakuNode, topic: Topic, message: Message) =
-  echo "NYI"
   ## Publish a `Message` to a PubSub topic.
   ##
-  ## Status: Not yet implemented.
-  ## TODO Implement as wrapper around `waku_protocol`, and ensure Message is
-  ## passed, not `data` field.
+  ## Status: Partially implemented.
+  ## TODO: Esure Message is passed, not seq[byte] `data` field.
+  let wakuSub = w.switch.pubSub.get()
+  # XXX Consider awaiting here
+  discard wakuSub.publish(topic, message)
 
 method publish*(w: WakuNode, topic: Topic, contentFilter: ContentFilter, message: Message) =
   echo "NYI"