nwaku API: push notifications on Relay and Filter API

[jm-clius]

Background

When the JSON-RPC API was implemented for nwaku nodes, one outstanding task was push notifications for new messages on subscribed pubsubTopics and contentTopics on the Filter and Relay APIs. This task was awaiting Nim websocket support, which has since been added.

Note, however, that with the introduction of a secure web server in Nim it is now possible to implement an HTTP REST API, which may be even more useful than the JSON-RPC API.

Acceptance criteria

• Ability to receive push notifications for new messages over a websocket connection in either the JSON-RPC API, a new HTTP REST API, or both.

cc @sbc64

[Ivansete-status]

I think that this can be closed as we have the REST API for relay as it is implemented and Zoltán will implement the filter one. @jm-clius , kindly close this if you agree.

[jm-clius]

The new APIs do not necessarily remove the polling requirement on client nodes. This issue can remain open to track work to convert from a polling to "push" model based on websockets.

[chair28980]

@jm-clius should we keep this issue open, or re-scope this work into 2024 milestones/epics?

[jm-clius]

The issue of a push vs polling interface has been open but unactioned for quite a long time. As it stands I think the polling API works well enough for us to descope this work (for now). I'd close this issue and reopen when the need arise