Rest API to access Lenra app

[taorepoara]

Define API endpoints to connect to Lenra app. The endpoints need to manage all what is possible through the websocket:

• get view data
• call listener
• load resource (I think that this is already possible)

The view response will return an Etag and/or some cache management elements in order to return a 304 status if there is no change and return a patch (if managed by the client).

This will permit the integration of Lenra in other tools like CLI or server side rendering apps (like NextJS).