Add Child only mode to SSE / Websocket

With Restconf API, it is possible to subscribe to event reported through SSE or Websocket.

Event is composed of time, data, path and type of event. Query parameter could be added to get leaf node only instead the whole data tree (see https://docs.opendaylight.org/projects/netconf/en/latest/user-guide.html#change-event-notification-subscription-tutorial)

However, for some use case (e.g got link event modification from network-topology or graph-topology) default query is too huge (the whole topology) and leaf node only too detailed (a link is composed of dozen of parameters).

This new feature introduce a new 'odl-child-node-only' query parameter when subscribing to the events to get in a compact JSON or XML only children that have been modified during this event. It could be considered as an intermediate choice between the default behavior and the 'odl-leaf-nodes-only' query parameter.