feat: simplify sending/receiving API

[fryorcraken]

This is a bug report/feature or change request/support request

Problem

From discussion waku-org/waku-react#1 (comment)

we can have the Light node and relay node implement an interface like that

interface WakuNode {
   sender: Sender;
   receiver: Receiver
}

Proposed Solutions

• Create ISender and IReceiver interfaces
• relay, filter, light push to implement these interfaces
• take the opportunity to align API for these protocols
• enables Waku node to also implement the interfaces and use lightpush+filter or relay under the hood (user only need to choose what protocol to use at node creation)

Notes

Other improvement:

Make filter.subscribe and probably relay.addObserver too accept a single decoder OR an array of decoder.

[weboko]

I believe it is done by #1219 and #1217 being merged

[fryorcraken]

Last check point is missing: having .send and .subscribe methods directly on the Waku interface.

[weboko]

Last PR to close the issue waku-org/examples.waku.org#239

[weboko]

Closed with the PR being merged