Add a streaming feature to the transfer process

[paullatzelsperger]

Streaming from the providing connector to the requesting connector should be implemented.

Option 1: client provisions data destination

Client creates the data destination, e.g. a web socket and sends the destination (credentials + address) back to the providing connector who will then start pumping data there.

Option 2: providing connector provisions data destination

The providing connector creates the data destination (e.g. a web socket) and sends its parameters (credentials + address) to the client, who can then "subscribe" (i.e. listen for messages).