[JS] Add realtime local clone

[vhdirk]

It'd be cool if the Ditto SDK could have a local clone of the remote digital twin, that is updated with websocket/sse. That way, the number of http requests could be a lot lower.

One example of this is the firebase firestore. The JS SDK can be set up to work like this, too.

What's your opinion on this? I'm willing to work on it, but it might take a while; it's quite a bit of work, and I don't have much time to spare.

[thjaeckle]

I like that idea very much. 👍
Would make building web uis showing live data from a twin really easy.

[tobias-zeptio]

I'd say this feature is great, but would probably not fit in the scope of a client. This is more of a application layer implementation, at least that how I have done it in the past. An implementation would depend on the javascript framework in use by whoever's using the library.

Maybe do an example implementation a as ditto-clients/javascript/examples/web-app.