Make the easyrtc adapter unreliable transport configurable

[vincentfretin]

From the discussion in #139
make the easyrtc adapter unreliable transport configurable (used for sendData and broadcastData) , so you can use websocket (like the wseasyrtc adapter) or a custom function to use a websocket from another backend instead of webrtc data channels.
With websocket transport you can do whatever you want on the server side to persist spawned entities.

In the janus adapter you can specify for reliableTransport and unreliableTransport "datachannel", "websocket" or a function to use another server. Example in the janus sendData code:
https://github.com/networked-aframe/naf-janus-adapter/blob/7840de8f2dbfd978136c7e1844d4677fbfd208fe/src/index.js#L1037-L1053